Why don't humans have tails or lay eggs? Adventures in evolutionary randomness.

A new study explains why humans don't have tails. It shines a bewildering light on the role of chance in the evolutionary trajectory of how we became ourselves—and why "good enough" often works.

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Why don’t we have tails?

It sounds like an absurd question, but it’s not. After all, our ancestors had tails. We, not them, are the weird ones.

If you were to take a family photograph that included our ancestors stretching back about 500 million years, this little guy—the Metaspriggina, a four-inch fish that slithered through the seas with a rudimentary tail, incipient jaws, and googly eyes—would be included. It’s one of our most distant relatives, a species that gave rise to vertebrates, including all of us.

Meet your long lost relative! I’m afraid you were cut out of the will, including the tail

We are primates. To this day, all monkeys have tails.¹ Most lemurs have long, flamboyant tails. But apes, including ourselves, don’t. At some point on our lineage of life, they disappeared.²

Now, by harnessing the wizardry of modern genomic research, scientists have finally been able to help explain what happened. But the reason for our lack of tails won’t just surprise you; it’ll also provide a window into a bizarrely random world of evolutionary change.

Much about what it means to be human can be chalked up to seemingly random forces—and the mystifying, arbitrary nature of chance, cracking some pretty big pillars of conventional wisdom in the process.

But when you ponder these seemingly arcane, abstract questions about why we are the way we are, it turns out there are profound lessons for modern societies, too.

Where did our tails go?

Twenty-five million years ago, there was an evolutionary split, separating primates with tails from those without them. That 25 million year-old ancestor is a relative that we directly share with monkeys, but since then, we’ve been on a different evolutionary path. We still have a physical leftover from that ancestor, our coccyx, or what most of call—fittingly—our “tailbone,” which extends below our pelvis.

But for one researcher, Dr. Bo Xia of Harvard, a chance event caused him to think more deeply about our missing tails. As he explained it to me, he was taking an Uber in May 2019, when he shifted to make room for an incoming passenger in the ride share. By accident, he sat on top of a seat belt buckle. For a year, his tailbone hurt, a constant reminder of his childhood fixation that he had often pondered as a kid: “Where is my tail?”

Several years ago, Xia stumbled upon the answer. For decades, it has been clear that tail development was at least partly related to a part of the genome known as the TBXT gene. It was also known that mutations within the TBXT gene can be associated with stumpy or short tails, as with the Manx cat, pictured below.

But until a few years ago, nobody had figured out what was going on within that gene. We were still in the scientific dark.

Until Bo Xia turned on the light.

He found 300 little letters. 300 out of the roughly 3.2 billion base pairs that comprise the human genome.³ For humans and other apes, they were identical, even in the same place within the TBXT gene. Xia wondered whether he may have found the exact moment—the evolutionary signature—of when we lost our tails.

And, in a new paper that was just published in Nature, the top scientific journal in the world, it appears Xia’s hunch was correct. The technical detail is fascinating, and is explained in this more readable account here, but the gist is that there’s a specific snippet of genetic data (known as an Alu element) that, when it occurs in tandem with another piece of genetic data, leads to the loss of a protein coding region (known as an exon). That subtraction is likely the origin story of why we don’t have tails.

But here’s where it gets most interesting.

Survival of the random, but not always the fittest

Many children wonder why we don’t have tails, and until recently, the best explanation was an adaptationist hypothesis. In other words, the reason we don’t have tails was that it helped our ancestors to adapt in order to survive long enough to reproduce. At its core, the overly simplistic adaptationist way of thinking is to look at a trait that currently exists, come up with an explanation for why it might have helped our ancestors survive, and then pat ourselves on the back for a job well done. It all makes sense! A neat and tidy story makes us feel good. Case closed.

The same logic was applied to tails. Scientists hypothesized that the loss of tails allowed humans to stand upright better, giving rise to bipedalism, separating us from those primates that clamber around on all fours, rather than walking tall on two legs.

But over the last decade, it has become increasingly clear that this explanation doesn’t match the actual data. For example, a 2016 study found that the few monkeys that do sometimes walk upright, such as capuchins, benefit from their tails for better balance even when walking upright. Then, in a 2019 study, a different set of researchers found that adding an artificial tail to human bodies actually provided better balance. (Here’s one heck of a set of photographs from that study, which is a pretty amusing example of science).

Then, there was another problem in the adaptationist narrative: it appears that our ape ancestors—those without tails—evolved to lose their tails while they were still living in trees. There’s no way that losing a tail was helpful for them. If you live in a tree, a tail is useful.

Bo Xia’s team has further obliterated the simplified adaptationist narrative. When they inserted the specific piece of genetic code that was associated with tail loss in apes into mice, they found that the mice either didn’t grow tails, or had short, stubby ones. But even more intriguingly, the loss of the tail for the mouse came with a significant cost: it created greater risks for an area of the body known as the neural tube, which is basically the spine plus the brain. That’s a big cost.

The evidence was mounting: tails were helpful, not harmful. The loss of a tail likely made it harder for the apes to survive, not easier. But how does that make sense in an evolutionary framework, in which mutations that are helpful tend to get passed down, on average, to the next generation?

The randomness of life

One possible answer is basically that it was all a bit arbitrary and random, a case of genetic drift. There was no grand purpose, no illuminating explanation.

It was an accident.

According to that theory, which remains speculative, a genetic mutation meant that an ape was born without a tail, sometime around 25 million years ago. That ape survived and reproduced, passing down the genetic mutation, and the lack of a tail to its offspring.

However, because of tectonic activity, climate change, and volcanic activity in East Africa at the same time, that nascent initial population of tail-free apes was cut off from other primate populations, isolated, evolving independently.

As I briefly explain in Fluke, such founder effects or population bottlenecks can amplify the role of genetic drift in which random variation sticks around, making chance a bigger force in evolution than natural selection.

Bottlenecks happen when genetic diversity plummets due to a sharp loss in the number of individuals alive in a species. For example, many people (myself included) have marveled at the northern elephant seals scattered across California’s beaches. But during the 1800s, humans hunted that species nearly to extinction for their blubber oil, until as few as twenty breeding pairs remained alive. Today, every elephant seal is a descendant of that small cluster. It isn’t hard to see how much it matters which individual seals survived to regenerate the species.

Now, imagine something similar happening with humans, in which the entirety of our species was whittled down to just forty people, before exploding to 8 billion individuals. The exact composition of those forty people would define the species. If all forty came from, say, nurses and doctors at a children’s hospital, future humans would turn out quite differently from what would happen if all forty were, God forbid, Kardashians.⁴ With such low numbers, every individual would reshape humanity. For better or worse, billions descended from a gene pool that began with one-fortieth Donald Trump would be rather different from the descendants of a gene pool that contained one-fortieth Malala Yousafzai instead.

When that happens, it’s more about survival of the luckiest, not necessarily the fittest.⁵

Similarly, it’s plausible that an isolated a group of primates who had lost their tails continued having offspring, generation after generation, passing down the no-tailed lineage. And that kept happening, all the way to us, as the engine of evolution chugged along and ended up producing tail-free Homo sapiens.

This upends the conventional wisdom: it’s not that humans don’t have tails because it helped us walk upright. Instead, it was an evolutionary mistake, one that actually hurt apes and humans, but that was locked into place by geographical isolation and the arbitrary forces of nature. Some apes got isolated, and poof! Now, we don’t have tails.

As Dr. Xia told me in an exchange last week, this sort of thing keeps happening, and there’s mounting evidence that random, seemingly arbitrary forces play a bigger role in evolution than was previously imagined. Adaptation still matters enormously, but sometimes…stuff just happens. There’s no eureka explanation. It’s just chance.

In fact, scientists even recently discovered that the reason why humans don’t lay eggs may be traced back to a single shrewlike creature that got infected by a retrovirus 100 million years ago, leading to placenta, and by extension, live births. But for one shrew, perhaps we’d all have hatched.⁶

The “good enough” theory and the harmful myths of “survival of the fittest” narratives

In 1856, the great naturalist Alfred Russel Wallace wrote that “Naturalists are too apt to imagine, when they cannot discover, a use for everything in nature.”

Daniel S. Milo, author of Good Enough: The Tolerance for Mediocrity in Nature and Society, convincingly argues that much that changes in the world—whether in human society or evolutionary biology—is neutral, sometimes driven more by a pressure that it just has to be “good enough” to survive.⁷ And in that framework, chance matters more than we think.

We, with our brains evolved to detect patterns, are allergic to that sentiment. As Milo observes: “The brain abhors neutrality. The human brain is a significance gland. It secretes meaning in abundance. It will read between the lines, manipulate data, delude itself—do anything to make sense of events.”

And randomness, by its nature, doesn’t make sense. Nor does the fact that apes lost their tails even though it probably made them less likely to survive. There’s no neat and tidy story there, just irreducible, arbitrary contingency. On the other hand, it does have the virtue of probably being true.

But there’s a deeper lesson here, too. The excesses of the adaptationist interpretation of evolution, in which everything happens for a reason, in which there’s a ruthless culling of inefficient traits from the world—”survival of the fittest” on steroids, if you will—is both wrong and can even be harmful as a social paradigm.

There are too many people who have taken a misunderstanding of evolutionary theory and mis-applied it to all sorts of social realms, from the grotesque extremes of eugenics to the justification of private equity firms that destroy lives as they cull what they see as non-optimized, inefficient excesses. The idea is that nature’s evolutionary process is, for some inexplicable reason, supposed to be a guidebook to how we should set up human society.

That’s wrong for at least two major reasons.

First, the forces of evolution aren’t morally good or bad; humans construct morality in how we choose to shape society and we aren’t constrained by how the natural world operates. We can imagine a world that’s different—and better—from that which has been observed in the past, which is one of the greatest and most unique traits within our species.

Second, it’s factually wrong. Evolution isn’t always a relentless optimizer, ruthlessly eliminating “excess” or “waste.” In fact, if that were true, we wouldn’t exist, because the tail-less apes would likely have been eliminated—and humans would never have emerged. Some of the ingenuity of evolution only works through diversity and experimentation, traits that are routinely downplayed by those who absurdly imagine the forces of evolution by natural selection to be a useful pretext to justify inflicting malicious harm in human society.

As Milo puts it eloquently:

“The pursuit of excellence is an admirable calling for some, but it is just one among many, including truth, faith, work, family, serenity, love, peace, pleasure, health, thrill, and fun. It is only because we…mistake natural selection for natural law that the lust for excellence continues to receive the imprimatur of nature, while these other pursuits are deemed epiphenomenal.”

So, next time you injure your tailbone, or whenever you contemplate what you might look like with a long, majestic monkey tail protruding from your backside, consider this: if optimization, perfection, and ruthlessness always prevailed as guiding principles in nature and in life, none of us would exist.

Thankfully, they don’t.

Everything doesn’t happen for a reason. But chance and randomness do make our world a fascinating place to inhabit. And, sometimes, it’s worth remembering a powerful lesson:

“Good enough” often is good enough.

Thanks for reading The Garden of Forking Paths. If you enjoyed this edition, or learned a thing or two you didn’t previously know, consider upgrading to a paid subscription to support my research and writing, as my work is exclusively reader-supported. And if you liked reading this, do consider forwarding it on to your friends, relatives, mortal enemies, etc., so they can read it too. It helps enormously. Now, enjoy the rest of your day, you beautiful tailless creature.

1. The Barbary Macaque is an exception, though it technically has a tail, just a really small one that’s mostly not visible externally, known as a vestigial tail.

2. Human embryos have tails until about eight weeks.

3. Okay, okay—it was 297 letters, not 300.

4. I’m partly joking about this; I’m not a genetic determinist, so genes only explain part of a person’s behavior, and probably less than many people imagine with the (very wrong) idea that genes are a “blueprint” for everything in our lives.

5. “Survival of the fittest” is one of the most misused terms in evolutionary biology and it’s also not something that Darwin coined; it was developed by Herbert Spencer.

6. Realistically, if that shrew hadn’t been infected, and placenta hadn’t emerged, it’s unlikely that something identical to Homo sapiens would exist. There’s a lot of contingency in evolution.

7. This draws on the work of Motoo Kimura, who makes a cameo in the opening chapter of Fluke.