What’s in it for me? A romp through the history of life, with surprising insights from the world of genetics.
Looking back on the story of evolution, the path seems relatively straightforward. First, fish evolved to walk on land. Then, reptiles grew feathers and learned to fly. Finally, primates evolved into humans who can do things like juggle, compose symphonies, and design spaceships.
But the journey from primordial soup to where we are today was far from direct. Paleontologists have long seen this in the fossil record. Now, thanks to scientific advances, we can use DNA to show the many twists, turns, accidents, and conflicts that make up the story of life.
We are closer than ever to understanding the history of life on Earth. But, as you’ll discover in these blinks, the truth is anything but straightforward.
In these blinks, you’ll learn
how fish were breathing air long before they evolved to walk on land;
that you have more in common with a sea squirt than you probably care to admit; and
why the salamander’s lightning-quick tongue was destined to evolve the way it did.
Plants and animals don’t change by developing new traits – rather, they repurpose old ones.
The question of how a fish could have grown legs and begun walking on land is one that has obsessed paleontologists for generations.
Charles Darwin’s contemporaries questioned his theory that evolution is a journey of intermediate stages, which is something the author – a paleontologist – wondered about as well. Why, for example, did the appendages that would eventually become wings appear on the ancestor of a bird that couldn’t fly?
Why would useless wings grow, rather than disappear? The answer is not that tiny wings grew into big wings. It’s that what we know as wings actually started out as something else. The author’s new mantra, inspired by Darwin’s response to his critics, is that nothing begins where you think it does.
The key message is: Plants and animals don’t change by developing new traits – rather, they repurpose old ones.
Darwin’s argument – that new features arise as a result of old ones changing functions – forever altered the way we see the history of life. To understand how, let’s take a look at the lowly fish.
In 1798, a certain fish, observed by a French scientist in Egypt, became famous. Why this particular fish? Well, it had unusual air sacs in its body. Now, most fish possess air sacs; they’re known as swim bladders, and they help fish remain submerged at a specific depth of water. But the famous fish’s air sacs were different. They were connected to the fish’s esophagus. They were proto-lungs.
Frenzied scientists began combing lakes and rivers all over the world for more data. Lo and behold, this Egyptian fish wasn’t actually that special. There were lots of fish with the same proto-lungs. Air-breathing fish weren’t the exception – they were more like the rule.
Recent research shows that proto-lungs and swim bladders are different versions of the same organ. And the genes that build swim bladders are the same as those that build lungs – in fish and, as we’ll soon see, in people, too.
Lungs aren’t a new invention that came about to help fish adapt to dry land. Rather, fish were breathing air long before they stepped out of the water. Evolution just repurposed the fishes’ air sacs.
See? Nothing begins where you think it does.
Embryos hold the secret to how change happens – for individuals and for species.
Scientists have long believed that embryos, and the process of transition from egg to adult, hold the secret to understanding the differences between species. But the first real breakthrough came in 1865, when French scientist Auguste Duméril peered into his aquarium and was met with a shocking sight.
Several months earlier, Duméril had introduced a species of salamander from Mexico. What confronted him now were two types of salamanders – the parents had given birth to what appeared to be an entirely different species.
The subsequent investigation into this seemingly bizarre occurrence had serious implications for the study of evolution.
The key message is: Embryos hold the secret to how change happens – for individuals and for species.
Duméril became obsessed with the mystery of the two salamanders. After suspiciously interviewing his household staff about whether they had tinkered with the aquarium, he set to work. He deduced that embryos for this one species of salamander had two possible developmental pathways. Which path they took depended on the environment in which they found themselves.
The original salamanders from Mexico had grown up in an aquatic environment. So, they had fully developed features to help them move through water – a fin-like tail and webbed feet. Their offspring, on the other hand, had grown up in a dry enclosure, so their development was arrested; they sprouted tails and limbs better suited to dry land.
Duméril’s realization might seem limited in scope. But it has ultimately enabled scientists to isolate the ancestor of all vertebrates, including fish, amphibians, reptiles, birds, and mammals.
It’s not necessarily an ancestor you’ll be proud to call your own. The mother of practically all animal life on Earth is the sea squirt, a lumpen creature that spends almost its entire life stuck to a rock.
But what the sea squirt lacks in glamor, it makes up for in genetic elegance. As an embryo, the sea squirt takes tadpole form, traveling freely through the ocean. After a few weeks, it loses its tail, nerve cord, gills, and nearly all of its connective tissue. It floats to the bottom of the seafloor and attaches itself to a rock.
The ancestor of all vertebrates came about because, eons ago, a sea squirt stopped its development early – just like the salamander. Instead of losing its nerve cord and connective tissue, for some reason, it kept them, and the individual grew into adulthood with its juvenile features intact.
These traits would eventually become our own.
Tiny malfunctions in the genome can have big consequences.
If learning the truth about your sea-squirt ancestors upset you, you might want to skip this next part.
You probably know that DNA is passed from parent to child. What you might not know is that, over the four-billion-year history of life, it has also passed from ancestral species to descendent species. That means you and a sea squirt share some of the same DNA.
Of course, unlike the sea squirt, you are intelligent, erudite, and a physical specimen of near-perfection – so how can it be that you share DNA with a strange blob on the seafloor? The answer lies in the differences between our genomes. By studying the ways genomes differ, scientists can learn about the relationships between species – even more so than by looking at fossils.
The key message here is: Tiny malfunctions in the genome can have big consequences.
DNA is made up of proteins in a specific sequence, which in turn are made up of thousands of amino acids – again in a specific sequence. A genome is the DNA of a plant or animal with everything in its right order.
Order matters to genomes. But how do genomes organize themselves? The answer is called a switch. Simply put, switches are instructions for when and where to add each part of the genome. This process starts while an embryo is forming inside its mother but continues into, and throughout, adulthood.
Switches are incredibly important because any tiny irregularity in the sequence can throw the whole thing off. For example, the difference between healthy red-blood proteins and sickle-cell ones is a single amino acid – but the consequences can be fatal. The author reports that almost 70 percent of people with sickle-cell anemia die before the age of three.
Sometimes, though, a malfunctioning switch can bring about a change that drives evolution. Recent research shows that changes in switches are behind every major shift in the evolution of animal bodies – from skulls to limbs, and fins to wings. These evolutionary transformations don’t mark changes to the genes themselves. They are changes in when and where the switch turns on.
So what does this say about the difference between you and a sea squirt? It’s not the difference in your genetic material that gives you a big brain and the ability to take offense. It’s in the switches that control how your genes do their job.
The mutation, or modification, of genes has triggered remarkable evolutionary change.
For Darwin, natural selection was the motor for evolution, and individual variation was therefore the fuel. But, of course, individual variation can also mean mutation.
Hearing the word “mutant” might trigger images of sci-fi Frankenstein monsters. But for Darwin it was a little less dramatic. A mutant is just an individual that has a trait that looks or behaves differently from the norm. If that trait makes the individual successful, they will pass it on to the next generation. If the trait harms their chances of success, it will likely die out over time.
There are two different modes of mutation. First, there’s variation in the size or degree of organs. Next, there’s the presence or absence of structures – like extra fingers or kidneys – which is variation of kind.
The key message is: The mutation, or modification, of genes has triggered remarkable evolutionary change.
By studying mutants, we can better understand how genetic information is passed down, and how evolution occurs. By studying mutations of fruit flies, for example, scientists realized that the structure of the body is mirrored by the position of genes on the DNA chromosome. The genes that are active in building the head are at the top of the chromosome, those for the abdomen are in the middle, and so on.
This in itself was a major discovery. But what happened next shook the science community to its core.
It turns out that this wasn’t just true for fruit flies – it’s the same for a huge number of animals. Flies, frogs, worms, and even people all have the same top-down, body-building gene sequence on our chromosomes. With this discovery, scientists could finally address questions of how animals developed and evolved.
Soon enough, they made another discovery. The same genes trigger the growth of limbs in a huge variety of animals – from frogs’ legs to the flippers of blue whales. Fish have them, too; only they grow fins, not limbs. And what’s more, these genes don’t just lie dormant in fish – they cause the development of the bones that sit at the end of the fins.
Copying, mutating, jumping, and even hunting are elemental parts of the human genome.
Given all that humans have been able to accomplish, you’d be forgiven for thinking that our bodies are far more advanced than those of our closest relatives – and certainly more so than fish or plants. But if the sea squirt teaches us anything, it’s that our arrogant pride at being the world’s top predator is unfounded.
Nowhere is this more obvious than in our genome. It seems reasonable to assume that our genome is a well-oiled machine where the chromosomes are in perfect order and everything works together as elegantly as a world-class orchestra. But that assumption is wrong.
Not only is the human genome an incredibly active place – its components are in constant battle with each other.
The key message here is: Copying, mutating, jumping, and even hunting are elemental parts of the human genome.
An animal’s complexity actually has nothing to do with the amount of its genetic material. A great example of this is corn, which has twice as much genetic material as you do.
In fact, much of animal DNA is what scientists call “junk” – the result of duplicated genes that are basically useless.
But the genes that are active in the genome are working hard, and in all kinds of unexpected ways. When sperm and eggs are manufactured, chromosomes divide and make copies of themselves. This is useful because the old gene retains its original function, and the new gene gains a new one. But somewhere deep in history, this process went haywire – when genes learned to jump.
That’s right – not only can genes copy themselves, they have the ability to hop around the genome, duplicating themselves wherever they land. According to Nobel Prize–winning American scientist Barbara McClintock, about 70 percent of our genome is made up of these jumping genes.
But in case you were starting to feel sorry for the normal DNA segments being taken over by these jumping upstarts, rest assured they are perfectly capable of defending themselves. Normal DNA segments have developed a way to go after jumping genes and literally weigh them down, stopping them from jumping to another location.
This mechanism helps control jumping genes, but allows them to continue replicating in ways that are beneficial to the genome – and the species.
Even if evolution had progressed differently, some outcomes would largely be the same.
As we’ve learned, the sea squirt is the common ancestor for all animal life on Earth. But billions of years before the sea squirt, another revolutionary event laid the foundation for life as we know it.
The earliest living organisms on Earth were single-celled creatures. Over time, some of these organisms started to produce oxygen as a waste product, and the Earth’s atmosphere began gradually filling with oxygen. After billions of years, a different microbe that took its energy from oxygen began to flourish. When these microbes teamed up with a bacteria that could produce complex proteins, the formation of bodies became possible.
For many years, scientists remained awestruck by this history – and what came later. Any tiny difference, they thought, would have thrown everything off. They were wrong.
The key message here is: Even if evolution had progressed differently, some outcomes would largely be the same.
To illustrate the surprising fact that some outcomes would have come about regardless, let’s return to our old friend the salamander. Maybe you’ve seen videos of a salamander catching a fly with its tongue. The tongue shoots out of its mouth so fast that, even in slow motion, it’s hard to see. Then it recoils into its mouth almost as fast. It’s truly a biological marvel – an incredible feat of evolution.
But here’s the thing: the different species of salamanders with projectile tongues aren’t closely related to each other. Their family trees are almost completely distinct. This means that the evolution of a projectile tongue happened more than once – at least three times, maybe more. And each evolution occurred independently of the others.
The reason for multiples like this is that there can only ever be a limited number of solutions to a problem. Take flying. Every creature that flies has wings rather than, say, helicopter blades because they need a big surface area to create lift.
If all animals use versions of the same genes to build their bodies, it shouldn’t be a surprise that we sometimes get multiples of the same thing. The arrival of great inventions like flight, or projectile tongues, is anything but an accident.
The history of life is not a crapshoot; the dice are certainly loaded. The ways we have evolved are affected by how genes build bodies, by the physical constraints of our environment, and by history.
Final summary
The key message in these blinks:
The history of life is a roundabout story full of dead-ends, wrong turns, and battles for genetic supremacy. But all animals have amazing and unexpected genetic similarities. And while we see some surprising evolutionary outcomes, it’s this interconnectedness that means some of those outcomes are perhaps not so outlandish after all.
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What to read next: Your Inner Fish, by Neil Shubin
Now that you’ve learned about the similarities between all animal DNA, take a deep dive into the 3.5-billion-year journey it took to get here. In the blinks to Your Inner Fish you’ll find out all about our evolution from fish to human.
See what fish can teach us about our bodies, discover why we look the way we do, and how we’re able to do all that we can do. If learning about our connection to the sea squirt was a revelation, try Your Inner Fish and see where that journey leads!