In part one, we looked at the roots of evolutionary theory—and a Turkish family whose kids seem to have lost the ability to walk upright. Can humans evolve backwards? More importantly, have we become “unnatural selectors” of our own future?
Devolution: fact or fiction?
Belgian biologist Louis Dollo was the first scientist to look at reverse evolution—and declared it impossible in 1905: “An organism never returns to its former state.” Over the decades, scientists have debated Dollo’s Law but most of the evidence—despite its complexity—seems to vindicate his claim.
Deceptive appearances: There are a number of examples of species seemingly going backwards. For example, birds. For over 100 years, scientists assumed that modern birds differ from their dinosaur ancestors in two key ways: a toothless beak and a mobile upper jaw. But a recent Cambridge study found a dinosaur that also had a mobile upper jaw. This suggests that a number of bird species with fixed jaws—like ostriches and emus—may have actually evolved “backwards.”
Point to note: Even human evolution hasn’t progressed in a tidy straight line. The brains of homo species progressively became bigger and bigger as we evolved—resulting in the homo sapiens, i.e us. Yet there is evidence of Homo floresiensis—whose brains were the size of a chimp—resembling a homo species that predated them by two million years:
Scientists’ best guess is that this species descended from a brawnier, brainer Homo species that got marooned on Flores and evolved its diminutive size as an adaptation to the limited food resources available on its island home. In so doing, H. floresiensis seems to have reversed what researchers once considered a defining trend of Homo’s evolution: the inexorable expansion of the brain.
Loss of complexity: Theories of ‘reverse evolution’ assumes that species move “backwards” when they lose a complex trait. This National Geographic article, for example, lists hagfish that “lost” their eyes, penguins that “lost” their wings, snakes that “lost” their legs. But as many experts point out, it is incorrect to describe this as “devolution”:
The notion that humans might regress or "devolve" presumes that there is a preferred hierarchy of structure and function—say, that legs with feet are better than legs with hooves or that breathing with lungs is better than breathing with gills. But for the organisms possessing those structures, each is a useful adaptation.
And the notion that evolution results in greater “complexity” is also wrong. A closer look reveals exactly the opposite:
For example, the lower jaw in vertebrates shows decreasing complexity, as measured by the numbers of bones, from fish to reptiles to mammals. (Evolution adapted the extra jaw bones into ear bones.) Likewise, ancestral horses had several toes on each foot; modern horses have a single toe with a hoof.
In other words, losing wings allowed penguins to become bigger and stronger in the coldest parts of the world. And that’s evolution not devolution.
No ‘reverse’ gear? A 2009 study also found that it is nearly impossible to reverse an evolutionary step—which involves a number of mutations. These typically involve a number of “restrictive” mutations that essentially block the path behind:
“I would never say evolution is never reversible,” Dr. Thornton said. But he thinks it can only go backward when the evolution of the trait is simple, like when a single mutation is involved. When new traits are produced by several mutations that influence one another, he argues, that complexity shuts off reverse evolution. “We know that kind of complexity is very common,” he said.
The future of humans—the ‘unnatural selectors’
Ok, we’ve now fully established the fact that humans are highly unlikely to start walking on all fours—and devolve into early primates. But what does our future look like? And will natural selection determine our fate—or have we become the “unnatural selectors”—shaping how humans and all other species evolve?
Unnatural selection? Scientists agree that humans are now the most powerful evolutionary force on the planet. Every other species has to adapt to live alongside us. And we are changing the environment so quickly that many of them cannot adapt fast enough to survive—hence, their extinction rate is 1000X faster due to human interference.
There are also a multitude of “human-induced trait changes”—observed in animals on every continent other than Antarctica. North American songbirds have shorter wings (deforestation), Zambian elephants are born without tusks (poaching) and sea snakes have darker bodies (pollution). As one scientist puts it: "There has never been another species that has so quickly changed the course of evolution."
Also this: Then there are human experiments in creating ‘living machines’. Example: Xenobots built from stem cells of frogs—which can create copies of themselves. We have not yet been able to engineer new species that can spontaneously evolve—but we’re getting there.
But are we humans also evolving? If so, what will we look like millions of years in the future? Here is a quick roundup of the key theories.
Homo stasis: Over a decade ago, a number of scientists still believed that human beings have stopped evolving. Their reasoning: new homo species evolved in isolation from one another:
Each new species evolved when a small group of hominids somehow became separated from the larger population for many generations and then found itself in novel environmental conditions favouring a different set of adaptations. Cut off from kin, the small population went its own genetic route and eventually its members could no longer successfully reproduce with the parent population.
That isn’t possible any more:
Since the advent of settled life, human populations have expanded enormously. Homo sapiens is densely packed across the Earth, and individuals are unprecedentedly mobile. In this situation, the fixation of any meaningful evolutionary novelties in the human population is highly improbable. Human beings are just going to have to learn to live with themselves as they are.
But, but, but: Genetic research shows that humans are indeed evolving—and at an accelerated pace. A 2007 study found that 7% of our genes mutated as recently as 5,000 years ago. And many of these changes were made to adapt to environments we created—for example: dairy farming, which explains why everyone in Sweden and Denmark can easily digest fresh milk while few in China or Africa can.
Force of nature: Other experts insist that natural selection is alive and well. The future will be shaped by who humans mate with—which ones have babies. Example: shorter and plumper women have more kids. Or maybe sexual selection will favour more intelligent humans:
You still have powerful mate choice shaping mental traits particularly … traits that are needed to succeed economically and in raising kids… We're also going to get stronger sexual selection, because the more advanced the technology gets, the greater an effect general intelligence will have on each individual's economic and social success, because as technology gets more complex, you need more intelligence to master it.
But, but, but: There are also controversial arguments that intelligence is an evolutionary handicap. Well-educated humans—especially women—increasingly choose to postpone having babies or choose not to have any: “If less intelligent parents have more kids, then intelligence is a Darwinian liability in today's world, and average intelligence might evolve downward.” That said, there is absolutely zero evidence that human intelligence is dipping downwards—which leads us to the next theory—’engineered humans’.
‘Engineered’ humans: Genetic technology will inevitably allow humans to artificially select their babies:
The pressure to change genes will probably come from parents wanting to guarantee their child is a boy or a girl; to endow their children with beauty, intelligence, musical talent or a sweet nature; or to try to ensure that they are not helplessly disposed to become mean-spirited, depressed, hyperactive or even criminal. The motives are there, and they are very strong.
And this may potentially lead to a new kind of class divide:
If the rich and powerful keep the artificial-selection technology to themselves, then you could get that kind of split between a kind of upper-class, dominant population and a lower-class, genetically oppressed population.
And if these highly engineered kids keep to themselves, we might end up here: “With some kind of self-imposed geographical or social segregation, their genes might drift and eventually differentiate as a new species.” Or not. Maybe genetic engineering will become as widely available as the mobile phone. Either way, it will most certainly shape the evolution of humans.
Digital immortals: Science fiction and futurists have long predicted the fusion of human and machine. Many scientists suggest that the Darwinian forces of evolution are vastly outpaced by the dizzying speed of technological change. For example:
In addition to living forever, "uploaded" beings would be able to "travel at the speed of light as an information pattern," download themselves into robots for the occasional stroll through the real world, think faster when running on advanced operating systems, and cut their food budget down to zero.
Of course, by fusing ourselves with machines, we could seal our own doom—as dystopian experts suggest: “Advanced artificial intelligence could encapsulate the various components of human cognition and reassemble those components into something that is no longer human—and that would render us obsolete.”
The bottomline: The only useful thing about humans is that we are the only species that can contemplate our own evolution—which offers some hope for us and the other creatures who share this planet with us. We hope.
Read quick takes from experts on devolution in Science Alert, LiveScience and Scientific American. New York Times has more on the study on “restrictive mutations.” BBC Future has an excellent long read on how humans have become a powerful evolutionary force. These older National Geographic (paywall) and Scientific American (login) deep dives lay out the various theories on the future of humanity.