Great fleas have little fleas,
Upon their backs to bite ‘em,
And little fleas have lesser fleas,
and so, ad infinitum.
~ Augustus De Morgan, A Budget of Paradoxes
You are not alone. Certainly, you have family and friends to help, support, and encourage you. But you have even more intimate partnerships.Of the trillions of cells that make up your body, only about half are human. What then are the rest? They are the numerous microbes that live on and in you.
We tend to think of microbes as harmful, causing everything from the common cold and flu to tuberculosis and malaria. But this is just a small subset of the microbes that are out there. Many are harmless, neither hurting nor helping you. And some are beneficial, even vital, not just to us, but to life on Earth.
Take the microbes that you carry around with you daily. Some are just along for the ride. But others play essential roles, ones you couldn’t live without. The microbes in your gut, for example, help you digest your food. Others help to train and instruct your immune system. Some of the residents of the skin keep away other, more harmful pathogens.
The collection of microbes in and on you is collectively called the human microbiome. You are a working, walking ecosystem, and your health depends importantly on these co-travelers with you. The microbiome is not static, but dynamic – it changes as you grow and is influenced by what you eat, whether you are taking antibiotics, where you live, and so on. There are different collections of microbes in different parts of your body, like your skin, nose, or gut. Even the two sides of your body have different groups of microbes.
A partnership between two organisms is a symbiosis, and a partnership where each benefits is a mutualism. Mutualisms are common throughout the natural world. Bees visit flowers, each one helping the other – the bee gets nectar and the flower is pollinated, allowing it to reproduce.
Cows eat grass made of cellulose, which they can’t digest. Within a specialized chamber in their gut called the rumen, they house bacteria that are able to break down cellulose into other molecules that the cow can digest. So, the bacteria and the cow both get a ready supply of food, and they couldn’t live without each another.
Some partnerships are even more intimate, where one lives in the tissues of the other. For example, certain bacteria live in the roots of some plants, where the bacteria get nutrients from plants. The bacteria in turn take nitrogen gas from the atmosphere, which is plentiful but unavailable in its form to plants and animals, and convert it to a form that is readily available.
A coral is a tight partnership between an animal and a photosynthetic alga. A lichen is a fungus and an alga that have taken a “lichen” to each other.
One of the more surprising discoveries of a tight partnership is a photosynthetic sea slug. A sea slug is a type of mollusk, which is an animal. How does an animal carry out photosynthesis, which we commonly associate with plants? The answer is that it has incorporated photosynthetic algae in its tissues.
In some cases, one organism lives not just within the tissues, but actually inside the cells of another. For example, aphids rely on certain bacteria that live in their cells to obtain the nutrients they need. There is even a species of mealybug that takes this one step further – within the cells of the insect is a type of bacteria, and within these bacteria is another type of bacteria.
When we think about evolution, images of struggle and competition might come to mind. But what these examples teach us is that cooperation has accounted for some of the key transitions in evolution.
Your cells, in fact, are the result of an ancient partnership, as inseparable as the aphid and their bacteria. Your cells contain mitochondria, involved in energy metabolism, which bear unmistakable signs that they were once free-living bacteria.
How did these bacteria end up in your cells? A long time ago (in this galaxy), an ancient cell engulfed a bacterium capable of carrying out a form of metabolism similar to modern-day cellular respiration. Instead of digesting this bacterium, the early cell incorporated it, allowing it to live within the cell.
Plants cells contain mitochondria as well, but they also contain chloroplasts, where photosynthesis is carried out. Chloroplasts too were once free-living bacteria that were incorporated into an ancient cell.
Taking a broader view, you depend not just on the microbes in and on you, but on many organisms that share this planet.
The oxygen you breathe comes from bacteria, algae, and plants. The food you eat comes from animals, plants, and fungi. The cycling of many essential elements, like nitrogen and sulfur, depends on microbes. Decomposition is the realm of all kinds of organisms, like bacteria and fungi. Microbes would do just fine without us, but we would not do fine without microbes. Life would grind to a halt.
The fields of medicine and biology owe a tremendous debt to other organisms too. A common technique in research is the polymerase chain reaction (PCR), which uses an enzyme called Taq polymerase isolated from a type of bacteria that lives in hot springs. The blue blood of horseshoe crabs is used to test for deadly toxins from bacteria in medical equipment, saving millions of lives. Our very understanding of biology – from cell division to physiology to cancer – depends on the use of model organisms, like bacteria, worms, fruit flies, and mice, to name just a few.
We study diverse organisms to understand long-standing questions in biology. Planaria, a type of worm, can be split in half and regrow the missing halves; your spinal cord can’t. What is it about planaria that allow them to regenerate? Naked mole rats don’t get cancer, but we do – why is that? Bears hibernate, shutting down their kidneys over long periods of time – how come they don’t get renal failure, as would happen to us?
It turns out that both are correct, but it depends on what you mean. If there are many fish of the same species, it’s “fish.” If there are many fish (fishes?) of different species, it’s “fishes.”
For example, if I see a tank full of goldfish, I might say, “That’s a lot of fish.” But, if I consider the ocean, I might say something like, “There are many fishes in the sea.”
Figuring out plural forms in biology can sometimes be tricky. One fungus, two fungi. One hippopotamus; two hippopotami. One nucleus; two nuclei. But one octopus, two octopuses, not two octopi.
This is because “octopus” is Greek, not Latin. To convert a Latin noun that ends in “-us” to its plural form, you usually change “-us” to “-i.” Because “octopus” is Greek, its plural form should technically be “octopodes,” but that doesn’t sound right and you almost never see it.
In this way, octopus is like platypus: it’s platypuses, not platypi or platypodes.
Louse becomes lice. Mouse becomes mice. But grouse doesn’t become grice. The plural of grouse is simply “grouse.”
Similarly, tooth becomes teeth and goose becomes geese. But moose doesn’t become meese. If you are referring to more than one moose, it’s just “moose.”
Like moose and grouse, there are many other organisms in which the singular and plural forms stay exactly the same: deer/deer, sheep/sheep, and even species/species.
What’s the plural of pancreas? Pancreases? No, it’s pancreata. Similarly, stoma becomes stomata.
Ovum becomes ova and millennium becomes millennia. In the same way, datum becomes data. So, be sure to write, “The data are interesting” not “The data is interesting” even though the latter might sound better and is even more commonly used. One day, it might even be considered completely acceptable.
That’s because languages are living and evolve, not unlike organisms themselves. Charles Darwin himself likened the evolutionary process to the way that languages change over time: languages with common origins are similar, but have also become distinct. Think of Latin and its “children,” including French, Spanish, Italian, and Portuguese.
In On the Origin of Species, Darwin wrote,
It may be worth while to illustrate this view of classification by taking the case of languages . . The various degrees of difference in the languages from the same stock, would have to be expressed as groups subordinate to groups; but the proper or even only possible arrangement would still be genealogical; and this would be strictly natural, as it would connect all languages, extinct and modern, by the closest affinities, and would give the filiation and origin of each tongue.
We can still see the roots of modern languages, as well as how they have changed over the course of time. In the case of English, its Germanic origins are clear, but it has also incorporated many words from other languages, including Latin and Greek, especially in the sciences. Although it’s easy to pluralize many English words by simply adding an “s,” the plural forms of Latin and Greek words are more complicated, as many of these examples illustrate.
I recently spoke to a scientist who said that while “fishes” is still used by some biologists, it’s going out of style in everyday English, providing another example of the changing nature of language. And that’s no fish story.
We are all, in a way, familiar with genetics. We know that children resemble their parents. We know that there are sometimes uncanny similarities among distant family members. And not a day goes by without some mention of genetics in the news – a gene is implicated in a disease; DNA testing is used to solve a crime; another genome is sequenced.
Yet we might struggle with certain details. What is a genome and why do we care about its sequence? What are genes and how do they relate to traits we see all around us? Why do some traits get passed on – brown eyes, red hair, high blood pressure – but not always, and sometimes in seemingly random ways?
All of your genetic material – your DNA – is contained in the nucleus of your cells. Cells are small, and the nucleus is even smaller. And yet it is within the nucleus that your genetic material is contained. It’s like the genie in Aladdin, who has “phenomenal cosmic powers” but “an itty bitty living space.” Your genetic material has enormous biological powers, but is packaged in the tiny space of a nucleus of a cell.
Today, we have a good understanding of genetics and powerful new tools to edit DNA, like CRISPR. Furthermore, we are learning that our experiences can not only affect us, but also be inherited by epigenetic mechanisms. In this way, the genie is truly out of the bottle.
This, I think, gives me three wishes.
Wish #1: We recognize that we are all mutants.
You inherit your genetic material from your biological parents. As a result, you have two copies of your genetic material in most of your cells. Each of these copies is a genome. So almost all of your cells have two genomes.
There are a few exceptions, like eggs and sperm, that have one copy, not two. This is because eggs and sperm are produced by a form of cell division (meiosis) that reduces the number of copies from two to one. Then, when an egg and sperm combine to form an embryo, the resulting cell has two copies again.
It’s this form of cell division that explains why you are genetically different from your siblings, even with the same parents. In the process of meiosis, the two copies recombine, and they recombine in a different way every time it occurs. As a result, all of the eggs or sperm from an individual are unique.
They are unique not just from one another, but also from all of the eggs and sperm in the world. Profoundly, they are different from all of the eggs and sperm that have ever existed in the entire history of life.
Then, when a sperm fuses with an egg, they create a unique embryo – also unique in the history of life. You are one-of-a-kind, a genetic instance.
The embryo is a cell that divides over and over to make the trillions of cells in your body. This type of cell division (mitosis) produces exact copies of your genetic material (except for rare mutations). The genomes in each of your cells are therefore more or less the same. This is why you can spit in a vial to obtain a sample of DNA. Your spit contains cheek cells, and each cell has two genomes, just like the ones in the other cells in your body.
DNA is composed of subunits (bases) repeated over and over. It’s the order of the bases that carries genetic information. So, when we “sequence a genome,” we are determining the order of the bases along a DNA molecule. In 2003, the human genome was completely sequenced.
When we say “the” human genome, we have to be a bit careful because, in fact, there is no such thing as the human genome. Humans, like all organisms, harbor lots of genetic differences (mutations). There are as many different human genomes as there are different people on the planet. In a way, then, we are all mutants.
Wish #2: We understand that we all have the same set of genes.
What then is a gene? Genes are stretches of DNA that carry out some function in a cell. Many genes code for proteins, which give a cell its structure and carry out much of the work of the cell.
All of us have the same set of genes. We may have different forms of these genes (alleles) due to mutations from one person to the next. For example, there are forms of genes that can predispose us to cancer, such as certain alleles of the BRCA1 and BRCA2 that increase the risk of breast and ovarian cancers.
Who has the BRCA1 and BRCA2 genes? The answer is that we all do – men and women, people with increased risk and people with decreased risk of cancer. The two genes are in fact essential genes; without them, we would not be alive. However, some people have particular mutations in these genes that make cancer more likely than in others; others don’t.
Wish #3: We won’t say “there is a gene for” common traits.
We have Gregor Mendel to thank for giving us an understanding of how these genes are inherited from one generation to the next. At the same time, the emphasis that we place on his studies sometimes has the unintended effect of reinforcing common misconceptions about inheritance.
Mendel worked with pea plants, focusing on traits like whether the pea is yellow or green, or round or wrinkled. By following one trait at a time and carefully counting the offspring of each cross, he was able to see patterns that up to that point were unclear.
He determined that there are factors responsible for the traits he studied. It’s these factors that today we call “genes.” He also realized that peas contain two copies of each gene, just like us. Different forms of these genes result in different traits, like yellow or green seeds.
Understanding Mendel’s studies helps us understand what genes are and how they behave. However, the traits that Mendel studied are much simpler than the ones we see all around us. Mendel studied traits that are influenced by a single gene. One form of this gene results in yellow seeds, and another in green seeds.
However, most if not all of the traits we are familiar with are not single-gene traits. Consider hair color, eye color, skin color, height, or weight – none of these results from variation in a single gene. Instead, they are influenced by many genes. Human height, for example, results from variation in thousands of genes.
But that’s not all. The environment also influences common traits. Human height, for example, is influenced by nutrition. As a result, it doesn’t make sense to talk about “a gene for” height because height is the result of variation in many genes interacting with the environment. The same is true for common traits we see all around us.
So, we shouldn’t say things like “there is a gene for” height, or for that matter high blood pressure, intelligence, or sexual orientation because, in fact, there isn’t.
A few summers ago, I collected photos of butterflies I saw in the Adirondacks in upstate New York. I didn’t intend to start this collection. It started innocently enough.
I was hiking with my 13-year-old son up Mt. Marcy, the tallest peak in New York state. We stopped for water (more for me than for him) when the two of us noticed a butterfly. It was a butterfly I had never seen before. I was curious to know what it was. It was brown with a broad orange stripe near the margin of each wing.
Worried that I would forget its colors and distinctive markings, I decided to do what we might all do these days – I pulled out my “phone” and took a photo of it. Later, I was able to compare the photo to pictures on the web. I still had trouble identifying it, so I sent the photo to a friend who is a Lepidopterist (a butterfly expert), and quickly got a reply – it was a Milbert’s tortoiseshell. It’s not a very rare or unusual butterfly, but I was right – I had never seen one (or even heard of it) before.
This got me started on my collection. I decided to photograph all of the different butterflies I saw that summer. Some were common – like the Monarch:
Others I recognized but didn’t know their names – like the White Admiral:
And others were new to me – like the Great Spangled Fritillary:
At the end of the summer, I made a postcard of all of the butterflies I photographed, and I included both their common and Latin names. I am not sure why I arranged them as a postcard. They just seemed to form a nice set and a record of what I saw that summer.
Stories emerged from the collection. My wife, noticing a butterfly on the postcard called the Question Mark, asked me why I didn’t identify that one. I replied that that is indeed its name. This orange and brown butterfly gets its name from the distinctive curved line and dot on the underside of each wing, resembling the punctuation mark. This story has become part of our family collection of stories.
I am proud of the postcard, but what I really like is the way that the first photo led me to an accidental collection and to a story that has become a part of our family.
It’s funny because that’s exactly how my collection of license plate photos began when I was much younger. I was at my grandfather’s 50th (!) college reunion at Cornell in Ithaca, New York. People came from all over for the reunion. Ithaca is beautiful, but what I noticed were all of the unfamiliar license plates. I grew up in Massachusetts and did not travel out of the Northeast very much. So, when I saw an Alaskan license plate, I did what perhaps any inquisitive 8-year-old child with a new camera would do – I took a picture of it.
That’s the only license plate photo I took while I was at Cornell, but it got me thinking. Wouldn’t it be fun to see if I could photograph all 50 license plates without leaving the Northeast? I set myself this odd challenge and got to work. Whenever my family would go to a mall, for example, everyone would go shopping, while I would scour the parking for new license plates, ones I had never seen before that I could add to my growing collection.
Of course, some were easy – the New England states and most of the states along the east coast. But others were more elusive, like small Midwestern states. The very last one turned out to be quite a challenge. I nearly finished the collection – 49 of the 50 states – and even got all of the Canadian provinces and some far-away plates, like England and France and – of all places – the United Arab Emirates.
But the one that took me several years after I found 49 states turned out to be . . . Idaho. I’m not exactly sure why. I always thought it had something to do with the size of the population and its distance form where I lived. But who knows.
I hung all of the photos on a wall in my bedroom, creating a license plate mural that grew over time. Along the way, I learned some interesting lessons. Obviously, I learned a lot about geography. And I learned a lot about people.
I remember a time when my family and I were in Martha’s Vineyard. I saw a plate I really wanted – I think it was Texas. I had my camera at the ready when I saw a man with cowboy boots approaching the car. My instincts told me that he was the owner of the car. Not wanting to lose this rare opportunity, I decided to simply tell him what I was doing and ask him if it would be ok if I photographed his license plate. He agreed, but asked if I wanted “a real cowboy” standing next to it. To be polite, I said sure, but asked if I could take two – one with him and one with the plate alone (which is the one that ended up on the wall of my room).
Looking back at the collection, there are other lessons. It is interesting to see how the plates have changed over time. And not just the plates. Each license plate is centered in the photo, with just a narrow frame that shows the front or back of the car. There, I can see a nice record of cars of the 1970s, including vintage VW bugs.
I also ended up learning many of the states’ slogans – “Oklahoma is OK” and “The Land of Lincoln” (Illinois) are two that stand out.
With the first photo of the butterfly or the license plate, I did not know where it would lead. In fact, I didn’t think it would lead anywhere at all. I had no plan in mind – just an opportunity that I seized. I am glad I did.
I suppose this is the lesson of the two first photographs, a lesson also captured in Robert Frost’s poem “The Road Not Taken.” In this poem, he fully knows “how way leads on to way.” Roads, after all, always lead somewhere.
In light of recent events, I think it’s important to remind ourselves how similar we are to each other, not how different.
If you look at your family tree, you first come to close relatives, then distant ones. Ancestry companies using the latest DNA technology can help you find relatives you didn’t even know you had or uncover branches of your family tree long forgotten.
If you continue to zoom out, you eventually come to the human family tree, including everyone alive today and your human ancestors.
How closely related are you to everyone else? You are about 99.9% genetically identical to all other people, or 0.1% different. This is a very small difference.
Our species is only about 300,000 years old. In this short period, there simply has not been very much time for us to become very genetically different from one person to the next. We are a young species, with relatively little genetic variation among different individuals.
This may come as a surprise because, if you look around, one of the first things you notice are human differences, not similarities. You might notice that hair color or texture varies quite a bit among people. Or eye color, nose shape, skin color, height, or weight.
We are particularly good at noticing differences among one another. In fact, we have grouped these differences into discrete categories called races. These races place people into just a handful of separate, non-overlapping groups based on outward appearances. And then we sometimes connect these superficial differences with much deeper ones, such as intellectual or athletic ability.
The racial categorization of humans, however, has no basis in biology. It is a social construct, used sometimes as a statement of identity, belonging, and pride, and sometimes to categorize, separate, and control people.
From a genetic perspective, we are simply not that different from each other. Many species, even ones whose members look quite similar to one another like fruit flies, harbor much more genetic diversity than we do. What this means is that external differences do not correlate well with underlying genetic differences.
In addition, there is actually no single trait (like a particular skin color or nose shape) that is universally present in one so-called race, but completely absent in another. In other words, it is impossible to come up with a trait that every member of one race has and that no one in another race has.
Finally, if we look at the amount of genetic variation within any race and compare it to the amount of genetic variation between any two races, we find something unexpected: There is much more genetic variation within a race than between two races. Put another way, there is more genetic variation within a group like Africans or Caucasians, than there is between Africans and Caucasians.
This doesn’t mean that “race” is not real. Race, as a social and historical concept, is certainly real and has very real effects. Racial disparities in health and economics, for example, are tangible effects of living in a racialized society.
This also doesn’t mean that there are no genetic differences among human groups. Some differences, like skin color, seem to be adaptive: ultraviolet radiation is necessary for vitamin D synthesis but can also cause damage, and skin color balances these trade-offs. Other traits, like eye and nose shape, may be sexually selected, as first proposed by Charles Darwin. There are also particular diseases (like Tay-Sachs) that are only found in some populations. However, these traits result from a very small amount of genetic difference and they don’t map on traditional racial categories.
The obvious physical differences are literally just skin deep. The same is true in other organisms. For example, peaches have fuzzy skin and nectarines have smooth skin. This dramatic external change is the result of a single change (mutation) in one gene. Peaches and nectarines are essentially the same fruit with different skins.
The various qualities that make us human – our thought, intelligence, athleticism, musical ability, language, and so on – are universal human traits.
We can learn even more about ourselves by looking at our recent evolutionary past. Our closest living relative is the chimpanzee. Humans and chimpanzees share a common ancestor that lived about 6-7 million years ago.
We have a lot in common with chimpanzees, but also a host of differences. Those differences evolved during the time we have been separated from chimpanzees. These include our big brains, allowing for tool use, language, and culture; opposable thumbs; ability to walk upright; and long childhood, allowing us to explore, learn, and play over an extended period of time.
One way we can trace these changes is to look at human-like fossils younger than 6 million years. Two particularly famous fossils are Lucy and Ardi. Lucy was discovered in 1974 in Ethiopia and is about 3.2 million years old. Ardi is older than Lucy, about 4.4 million years old, but is a more complete fossil. She was discovered in 1994, also in Ethiopia. Both are hominins – early human ancestors.
Lucy and Ardi lived in Africa. Since the split with chimpanzees, there were several migrations of hominins out of Africa to the rest of the world. Some of our human ancestors leaving Africa came across another hominin group known as Neanderthals. Neanderthals, with their heavy brow and stocky frame, have entered popular culture with sayings like, “Don’t be such a Neanderthal.”
The Neanderthals lived in many of the same places and at the same time as the line that led to modern humans. Furthermore, there now is clear evidence of interbreeding between the two groups. It is estimated that 1-4% of the DNA of many of us is Neanderthal and responsible for some of our traits, such as our ability to fight certain infections, but also a predilection for certain diseases.
Tracing our evolutionary history allows us to come back to the question of how we are all related. Humans have very little genetic variation, and now we can say that, of this little amount, most of it can be found in Africa. This is because the groups of early humans that left Africa only carried with them a subset of the genetic variation present in the original African population.
In addition, because early humans encountered Neanderthals outside of Africa, only descendants of these early travelers, like modern Europeans, carry Neanderthal DNA. Africans don’t because their ancestors didn’t interbreed with Neanderthals.
The history of our species is one of repeated episodes of migration, mating, and mixing. As a result, just as there are no pure “races,” there are also no pure “groups,” such as British, French, or Germans. The only groups that might be able to claim a long history with relatively little intermixing are some Canadian and Alaskan Native American tribes and Australian Aborigines.
Like any family, the human family has a messy and complicated history, but it helps us to understand that, in the words of Maya Angelou, “We are more alike, my friends, than we are unalike.”
When I was a teenager and young adult, I always looked forward to reading Chet Raymo’s column called “Science Musings” in The Boston Globe. Chet Raymo is Professor Emeritus of Physics at Stonehill College in Easton, Massachusetts, and a well-known science writer. His short essays are reflections on science, education, and the natural world.
One of these essays, from the mid-1990s, made such an impression on me that I clipped it out and filed it in my “Science Education” folder, where I keep articles related to science and teaching. The essay is titled “Teaching a Sense of Wonder.” Here, Raymo makes a plea to 6th-grade science teachers, asking them not to emphasize terms and facts, but instead to stand back and think about what every middle school student should learn in a science class.
He boils it down to five important concepts, one of which is the history of life on Earth.
He writes, “Roll out a paper timeline in the longest corridor of the school. Start with Day One, the formation of the Earth. Walk across 3½ billion years of life, most of the way down the corridor, before encountering anything but microbes. Give the dinosaurs their few feet of time. Find our sliver at the end of the line.”
I have taken his message to heart. I teach introductory biology at Brandeis University, and I always take time to teach the history of life on Earth. Many websites convey this information in colorful ways. Some show the timeline linearly, others as a spiral. One particularly compelling website starts with a line representing today, then this month, year, century, millennium, epoch, etc., all the while keeping track of where we are on the growing timeline. Take a look. It’s dramatic and humbling at the same time.
Let’s take a quick tour here. The Earth formed about 4.6 billion years ago. For many of us, that’s a hard number to really grasp. String together 4.6 billion paperclips. They will circle the Earth about four times. That’s the age of the Earth.
For the first half billion years, there were no rocks and no life. But as soon as rocks formed, there is evidence of life. Scientists aren’t entirely clear how life emerged from non-life – it’s possible that early chemical reactions produced the first organic molecules, or that a meteor seeded the Earth with its first organic molecules – or even whether life exists on the many Earth-like planets that have recently been found in our galaxy. But multiple lines of evidence suggest that all living species on Earth can trace their ancestry back to a single living organism. We are all related, just on different branches of one big family tree.
What did the first life look like? It was single-celled, and lacked a nucleus, where the genetic material is housed in our cells. Modern descendants of these early life forms include two of the three great domains of life – Bacteria and Archaea.
The first two billion years of life on Earth (almost half of it) belonged to these single-celled organisms. It was a time of great diversification. Yes, the bacteria and archaea remained unicellular, but they evolved all kinds of different ways to harness energy.
One group of bacteria – cyanobacteria – evolved the ability to carry out photosynthesis, using the energy from the sun to build sugars and producing, as a byproduct, oxygen. So, for the first time, oxygen appeared in our atmosphere. Although we take it for granted today and can’t live without it, at the time, it represented a major crisis for life on Earth, earning the name “oxygen catastrophe.”
No other group evolved this ability – ever – in the history of life on Earth. It evolved exactly once. Organisms alive today, like plants and algae, that are capable of oxygenic photosynthesis can only do so because they incorporated, in one way or another, these bacteria.
The next major stop on our journey is the evolution of the third domain of life – the Eukaryotes – with cells that have a nucleus, like our own. This event occurred about 2 billion year ago, over half of the way along the timeline. These cells are the ancestors of our cells, but the creatures, like the Bacteria and Archaea that preceded them, were all single-celled.
It took another billion years (we’re at about 1 billion years ago, or 80% of the way down the timeline), before cells started getting together to form first simple and then more complex associations.
Unlike the evolution of life and the evolution of eukaryotes, multicellularity didn’t evolve once, but several times independently. For example, it evolved one time in the line of organisms that led to modern-day plants, and it evolved separately in a different line of organisms that led to modern-day animals. Put another way, the common ancestor of plants and animals was unicellular, not multicellular.
The last half billion years, the last 10% of the timeline, starts to look more familiar, and events happen quickly. The first marking point is the Cambrian explosion, a sudden appearance in the fossil record of many marine creatures. Don’t forget, all life is still in the water, where it first began.
If we divide the last half billion, or 500 million, years into five equal parts, we see the first fish around 500 million years ago,
the first amphibians around 400 million years ago,
the first reptiles around 300 million years ago,
and the first dinosaurs and mammals around 200 million years ago.
And what about us? Modern humans evolved just 200,000-300,000 years ago in Africa. In other words, everyone alive today can trace his or her ancestry back to Africa not that long ago. If the history of the Earth were a minute, we wouldn’t even get a second of time. No matter how you look at it, it’s a mere blink of the eye.
The writer Anthony Doerr in his two-minute entreaty imagined the history of Earth as the length of your arm, starting from the shoulder and working toward the fingertips. He asked, “And you? Your grandma’s toffee bars, your CD collection, your treehouse, your best-ever Halloween costume, every regret you’ll ever have, every dream you’ll ever dream, every mouth you’ll ever kiss (or wish you had)—they’ll all ride the microscopic edge of your fingernail, a tattoo so thin you’d need an electron microscope to glimpse it.”
In the course of teaching this material and trying to bring it to life in various ways, I have really come to appreciate how profound it is to understand the rich history of life on this planet and our tiny place on this long and dramatic timeline.
A newly described dinosaur named Patagotitan mayorum holds the record for the largest animal ever to have lived on land. Does its size matter?
One of the most dramatic but underappreciated aspects of life on Earth is the incredible range of sizes among organisms. The smallest free-living organisms are bacteria, specifically Mycoplasma. The largest are blue whales. In fact, the largest organisms to have ever existed on Earth are blue whales.
Mycoplasma are about 200 nanometers long, or 200 billionth of a meter; blue whales are about 30 meters long. That’s a difference of 13 orders of magnitude, where one order of magnitude represents a 10-fold change in size. In other words, the largest organism that ever lived is 1013 times larger than the smallest organism.
By mass, blue whales differ from mycoplasma by about 21 orders of magnitude. That is, a blue whale is 1021 times heavier than a mycoplasma.
No matter how you measure them, organisms differ tremendously in size.
Where are you and other humans? Regardless of how tall you are, you are somewhere in the middle. And, it turns out, your size influences so many other aspects about yourself – how you move about, what you can see, how you gain and lose heat, how you obtain oxygen from the air and nutrients from your food, even how long you live. In short, your size affects just about everything.
An ant can’t be as a big as you and still look like an ant. Conversely, you can’t be as small as an ant and still look like you. Why not?
Let’s start with a big ant. If an ant increases in size but keeps the same relative proportions (that is, it maintains the same shape and still looks like an ant), it would buckle under its own weight. Weight increases as the cube of the scale factor because weight is proportional to volume. However, strength of the legs increases as the square of the scale factor because strength is proportional to the cross-sectional area of the legs. In other words, the ant’s weight would quickly outstrip the amount of weight its legs could support, and the ant would come crashing down.
Now let’s consider what happens if you were as small as an ant, like Marvel’s Ant-Man. As you get smaller, you have relatively more surface area than volume compared to when you are bigger. Volume decreases as the cube of the scale factor, whereas surface area decreases as the square of the scale factor. So, volume decreases much more quickly than surface area.
You dissipate heat in part through the surface area of your skin. And heat is generated by many chemical reactions that make up your metabolism, which tracks with volume. In other words, with relatively more surface area than volume, your small self loses much more heat than you can generate, unless you speed up your metabolism, eat all of the time, and run around constantly.
The problem in both of these scenarios is that the ant and human kept the same shape, but changed in size. Neither works because an ant looks like an ant in part because of its size. And you like you in part because of your size.
When we imagine large ants and small humans, we think of them as maintaining the same basic shapes. This is known an isometry (“same measure”). Similarly, when certain kinds of salamanders grow, they keep their same basic shape. In other words, an older salamander looks like a scaled-up younger salamander.
A different way to think about scaling is to consider shape changes that occur along with size changes. This is known as allometry (“different measure”). One of the best-known examples of allometry occurs as you grow up. A baby’s head is quite large compared to its body. By contrast, an adult’s head is relatively small compared to its body. Your body, in fact, grows much faster than your head. So your shape changes as your size changes.
The same is true of the growth of many animals. The fiddler crab has one large claw and a second small claw. They begin about the same size, but one claw grows much more quickly than the other claw. So, an adult fiddler crab looks quite different from a young fiddler crab.
We can consider scaling relationships that occur as organisms grow, asking whether they maintain the same shape, like salamanders, or not, like humans. In addition, we can look at different organisms and ask how their sizes and shapes compare to one another.
As early as the 1600s, Galileo Galilei noticed that the bones of larger animals are not scaled-up versions of the bones of smaller animals. Instead, the bones of larger animals are disproportionately wider than those of smaller ones.
J.B.S. Haldane, a British scientist with wide-ranging interests, wrote, “Comparative anatomy is largely the story of the struggle to increase surface area in proportion to volume.” What he meant is that as organisms get bigger, various traits and functions (like weight) track with volume and increase quickly, whereas others (like strength of bones) track with area and increase slowly. As a result, any trait or function that depends on surface area will become modified to keep up with volume so it doesn’t lag too far behind.
Think of the lining of your gut. It’s one large surface area. If your gut were a simple tube, there would not be nearly enough surface area to take in nutrients to supply your body (a volume) with the energy and nutrients it needs. It turns out that your gut is not a simple tube. There are folds that increase the surface area of the lining gut. Along these folds, there are smaller folds, called villi. And, along the villi, there are still smaller villi, called microvilli. This is why the lining of the intestine earns the name “brush border,” resembling a 1970s shag rug.
Seeing where you are in size compared to other organisms not only helps you understand basic aspects of your anatomy, but also has practical applications. In the 1960s, scientists injected LSD into an elephant at the Oklahoma City Zoo to study its behavior. The elephant immediately became agitated, fell over, and died.
The dose of LSD was more than 1,000 times a typical human dose and the largest dose ever given to a living animal. It was calculated by scaling the dose given to cats based on weight. The authors concluded that elephants are very sensitive to LSD. Further examination, however, revealed that the problem was not the sensitivity of the elephant to LSD, but instead the dose of the drug.
Just as adults are not scaled-up babies, and a big ant and small human don’t work, an elephant is not a scaled-up cat. Organisms differ in many ways, but size is particularly important because, in fact, one size doesn’t fit all.
Yesterday, thousands of people marched to bring attention to climate change worldwide. Here, I thought it would be useful to state the facts, simply and in one place. What is the evidence for human-induced climate change?
Is the Earth getting warmer?
Yes. This is not an opinion; it’s a measurement. The surface temperature of the Earth has increased about 1 ˚Celsius (1.8 ˚Fahrenheit) since measurements first began in 1880. The year 2016 was the hottest year on record, breaking the previous record set in 2015, which broke the previous record set in 2014. In fact, 16 of the 17 warmest years have occurred since 2001. None of this is debated.
Are levels of carbon dioxide (CO2) in the atmosphere increasing?
Yes. This too is a measurement. In 2013, the amount of CO2 in the atmosphere rose above 400 ppm (parts per million), and it continues to increase each year. Although the amount of CO2 in the atmosphere fluctuates seasonally and over longer time frames, it has remained under 280 ppm for at least the last 10,000 years. This is not controversial.
Is CO2 a greenhouse gas?
Yes. This is based on basic laws of chemistry and physics. A greenhouse gas acts like panes of glass in a greenhouse. It lets in solar radiation and then traps heat re-emitted from the Earth’s surface. Without greenhouse gases, the Earth would be much colder than it is now. Adding greenhouse gases, such as CO2 and methane, to the atmosphere increases the amount of trapped heat and therefore warms the Earth. Some of the additional CO2 in the atmosphere is absorbed by oceans. There, it acidifies the water, which, along with increased ocean temperatures, is wreaking havoc on coral reefs and other marine life.
Is the increased CO2 the result of burning fossil fuels?
Yes. In part, we know this from the simple observation that we started burning significant amounts of fossil fuel at the same time that CO2 levels in the atmosphere began to rise, in the mid-1800s during the Industrial Revolution. However, this is a correlation, and correlation is not causation. Measurements of carbon isotopes implicate the burning of fossil fuels more directly. Isotopes are different forms of the same element with different numbers of neutrons. Most carbon atoms (99%) are 12C with 6 neutrons; about 1% are 13C with 7 neutrons; and an extremely rare form is 14C with 8 neutrons. Different sources of CO2 have different ratios of these isotopes. Scientists therefore measured the ratio of these isotopes in the atmosphere and found that the increased CO2 in the atmosphere comes from the burning of ancient organic matter (fossil fuels like coal, oil, and natural gas) and not from other sources such as volcanoes.
So, what’s uncertain and therefore open to debate?
We know that the Earth is getting warmer, the amount of atmospheric CO2 is increasing, and human activity is causing the increase. What we don’t fully know is what will happen to the climate over time and how different parts of the world will be affected. This is where models come in and all models come with a degree of uncertainty. Most of the climate models indicate that the Earth will warm 2-6 ˚C (4-12 ˚F) in this century.
So, the question is not whether the climate is changing or what is causing it. The questions are – To what extent will it change? What areas will be most drastically affected? And, most importantly, what can we do about it? These are the key questions that scientists and informed citizens, like the ones who took to the streets yesterday, are focused on.
Every summer, my family and I spend time in the Adirondacks, where we rent a log cabin by a roaring brook. We open the cabin to many friends – children and adults – like one big family.
Last summer, one member of our extended family was Oliver, a tall 17-year-old boy with a mop of curly brown hair and wry sense of humor. He had spent the previous two weeks on a school trip to Belize, and was excited to be back in the Adirondacks, where he and my sons lead hiking, canoeing, and rock-climbing trips in the High Peaks region.
One day during his stay with us, Oliver showed me a lump on his arm. It looked like a typical bite, except that it was swollen and had a bit of dry blood at the top. The lump itself was red, but there were no other worrisome signs – no red streaking up his arm, fever, pain, or itchiness.
At first, we weren’t concerned, thinking it would go away on its own. But it didn’t. It didn’t get worse, but it didn’t get better.
Perhaps it was infected, so we kept it very clean. Every night, we washed it with water, hydrogen peroxide, or iodine, then put on some antibiotic ointment and covered it with a Band-Aid or gauze.
Still it persisted. There is a small emergency room not far from the cabin, so we thought about having it examined there. But each morning, we thought it was getting better, or at least not worse, so we waited.
Just about then, we heard from his mother, Emily. She told us that a counselor on the trip had a lump on her arm from a spider bite. That made a lot of sense. Of course, Oliver must have the same thing.
One night, Oliver was tired from a long day of hiking and getting a bit worried about the lump. He kept washing and looking at it, when he thought he saw something coming out of it where it was open at the top. We all looked, but couldn’t convince ourselves that we saw anything. Maybe a bit of motion? Something alive?
Because he was going home in a day or two, we decided not to go to the emergency room. When he got home, he went to the local doctor. At first, everyone was stumped too. It looked like an abscess (a walled-off collection of pus from an infection), so they lanced it. When no pus came out, they began to suspect that this was not an infection or even a spider bite. A little research uncovered what was actually going on.
It turns out that botflies are not uncommon in Belize. The adult botfly lays eggs on a fly, mosquito, or tick, where they hatch into larvae and drop onto a human host. The larva burrows under the skin, where it grows. After several weeks, the larva emerges, drops to the ground, and pupates. Eventually, the adult emerges and goes on its way.
A botfly adult
What Oliver saw moving that night were the mouth parts of the growing larva. When he washed it, the larva was deprived of oxygen, so it came out for some fresh air.
Here’s how Emily described what happened next in an email after the mystery was solved:
Hi guys! I thought I’d give the complete story…Oliver went to the doctor’s office on Monday. Of course no one there had ever seen a botfly, but they confirmed that it was in fact a botfly. All of the nurses gathered around to see this exotic bug in their office! They sent him home hoping that covering it with plastic would deprive it oxygen so that it would sneak out enough that Oliver would be able to get it out at home with tweezers. No such luck, so back he went to the doc on Tuesday. A new bevy of nurses hovers around in fascination. This time they put double amounts of plastic on it to make sure it is fully deprived of oxygen so that in 24 hours he can return and hopefully this time it will in fact have been so deprived of oxygen that it will move out towards the air when they remove the plastic. Day 3 at the docs, and that is exactly what happened. They tweezed out Angelica, who now resides in a specimen jar in Oliver’s room….
When Emily first found out what it was, she texted, “Gross, eh?” She’s right – it is gross.
But is it? Or is it just disgusting from our point of view? The botfly is, after all, just trying to get by, like all of us.
Charles Darwin used these sorts of seemingly abhorrent animal behaviors to make an unexpected argument for evolution by natural selection. He described birds that eject eggs from other birds’ nests, killing them; ants that raid other ant colonies, enslaving the captives; and wasps that lay their eggs in caterpillars, where they hatch and eat the caterpillar alive. Then he asked – is it better to think of these behaviors as the result of a natural process or divinely created? Here is what he wrote in On the Origin of Species –
Finally, it may not be a logical deduction, but to my imagination it is far more satisfactory to look at such instincts as the young cuckoo ejecting its foster-brothers, ants making slaves, the larvae of ichneumonidae feeding within the live bodies of caterpillars, not as specially endowed or created instincts, but as small consequences of one general law.
It’s interesting that none of us thought to do some internet research while we were there. As Emily pointed out, a simple search turns up botfly right away:
Oliver googled something like “common insect bites in Belize” and immediately “botfly” came up. It made complete sense, from the look of the bite to the fact that monkeys often get them in the jungle, and Oliver had spent a hot night sleeping outside of his tent in the jungle.
Perhaps we didn’t look it up because we thought we knew what it was. Emily continues,
Occam’s razor suggests that the simplest explanation is usually the correct explanation. But in the case of Oliver and the botfly, that proved not to be the case: the simplest explanation was that he had a spider bite as did another member of the crew who went to Belize. Thus it would seem that Occam’s razor didn’t help because in fact it was not the simplest explanation. However, perhaps we need to add that simplest explanation is usually the correct explanation if you have all of the information.
Or perhaps it was because we were in a vacation mindset where we didn’t immediately turn to Google to answer all of our questions. In the Adirondacks, we don’t have easy access to the internet. We can go to a local library with Wi-Fi, but we didn’t even think to use it.
We were truly in a different world, which is what vacation is all about.
A few summers ago, I was driving my two sons to a trail head for a hike in the Adirondacks in upstate New York. They were going with a group of kids up a mountain called Iroquois.
I advised them to be sure to bring lots of water for the long hike. In turn, they asked me how I was going to spend my day. “Writing about urine,” I replied.
“Urine?” they asked incredulously.
I am an author of an introductory biology textbook. That day, I started writing a chapter on the kidneys, which produce urine. One of the many challenges that I encounter is how to make the material interesting and engaging to students of many backgrounds and experiences.
Since my kids showed some interest, or at least disbelief (which I took as a sign of interest), I decided to pursue the topic with them to see if I could find a hook or something that would engage them that I could then use to begin the chapter.
“How do you think I should start?” I asked them.
I tried a few possibilities. “Did you know that the kidneys don’t just remove waste, but also are important for water and salt balance?”
That didn’t do anything for them.
“Did you know that the kidneys produce a hormone that promotes red blood cell production?”
I clearly wasn’t getting anywhere with them.
“Did you know that when you break down proteins and other molecules, you end up with ammonia, which is toxic. Ammonia must either be excreted, or converted to something that is less toxic and then excreted?”
They started putting on their headphones. I had to think quickly.
“Ok, how about this? Have you heard about Gatorade?”
They paused and looked up.
“Well, Gatorade was invented at the University of Florida in the 1960s as a new sports drink to help (aid) the football team (the Gators). Instead of just using water and sugar, the researchers realized that electrolytes, like sodium and potassium, are also important, since water and electrolytes are both lost in sweat. Some credit Gatorade with the Florida Gators’ win over their rivals, the Georgia Tech Yellow Jackets, and the rest is history.”
“Not bad, Dad.” Then the headphones went on. But at least I had them for a couple of minutes.
So that’s how the chapter starts.
It’s not a bad beginning, but as I began to read about the kidneys and think about the chapter, I became more and more fascinated with the kidney itself. In many ways, it doesn’t need a hook at all.
What’s so interesting about urine? We see it several times a day, but most of us don’t give it any thought. But it’s quite fascinating. Really.
Consider the job that the kidney does. We have two ways to eliminate wastes from the body – some in the form of feces and some in the form of urine. Why two and not one? And aren’t they about the same?
No, they aren’t. If you think about it, you will note a key difference. The gut is a long tube that courses through the body and is open on both ends. In other words, it is continuous with the outside world. Therefore, it is relatively easy to take in food, extract what’s needed, and then simply leave the rest behind, so to speak.
The kidneys have an entirely different task, and a different challenge. They remove wastes that end up in the blood, inside the body. How do wastes get into the blood in the first place? They either come from food we eat, absorbed by the gut into our bloodstream. Or, they are a breakdown product of proteins and nucleic acids. Ammonia is formed, which in mammals like us gets converted to urea and excreted in urine.
In addition to wastes, the blood has all kinds of important components, like red blood cells to deliver oxygen to tissues, cells and antibodies of the immune system to fight infections, proteins for blood clotting or transporting other molecules in the bloodstream, and sugar for energy. All of these ingredients that the body needs get mixed up with wastes that must be removed.
So the task of the kidney is to separate these essential elements from the waste, the wheat from the chaff. That’s a tricky business.
You might imagine that the simplest way to do this is to actively secrete the wastes into the ducts of the kidneys, leaving all of the good stuff behind. While the kidney does do this to some extent, this isn’t the primary way that wastes are excreted.
Instead, the kidneys go about excreting waste in what might seem like a counterintuitive way. There is a filter between the blood and kidneys. This filter is, well, a filter, so it lets some things go through but not others. It blocks large proteins and cells, but lets through everything else. Everything else includes wastes, but also water, electrolytes, sugar, and other key substances.
In other words, it is not a very good filter at all. It’s bit like a strainer you might use to remove pulp from orange juice. Yes, it removes the pulp, but what you have when you are done is still orange juice.
Then what? We now have a filtrate of blood that includes both wastes and essential elements. So the kidney now has to reabsorb all of the essential elements back into the bloodstream.
Why go through all of this trouble? Wouldn’t it be simpler to secrete just the waste, and hold on to everything else? Yes and no. It seems simpler, but it might not work as well, for two reasons.
To secrete waste, you would need to be able to recognize it and pump it out in some way. This is fine if the waste is something you are already familiar with, like urea. But what happens if it’s something novel, something you haven’t seen before, like a poison from a mushroom you just ate? In this case, our kidney has an advantage. By basically discarding everything but then reabsorbing the things we need, the kidneys are able to get rid of any substance that the body doesn’t want or doesn’t recognize.
Another reason goes back to my first response to my kids. The kidneys are remarkable in that they don’t just excrete waste. They are involved in lots of other functions, one of which is the control of the amount of water and electrolytes in the body. By letting them pass through the filter more or less freely, but then selectively reabsorbing them, we can fine-tune the amount of water and electrolytes in the bloodstream, which is important for hydration, blood pressure, and the like.
Pretty interesting, isn’t it? Or maybe I should just stick with the story of Gatorade?