Charles Darwin's On the Origin of Species, published 150 years ago Tuesday, opened the book on our evolutionary past, which has since been traced by scientists back to fossil apes.
But where is evolution taking us? Will our descendants hurtle through space as relatively unchanged as the humans on the starship Enterprise? Will they be muscle-bound cyborgs? Or will they chose to digitize their consciousnesses—becoming electronic immortals?
And as odd as the possibilities may seem, it's worth remembering that, 150 years ago, the ape-to-human scenario in On the Origin of Species struck many as nothing so much as monkey business.
Human Evolution Is Dead
"Because we have evolved, it's natural to imagine we will continue to do so, but I think that's wrong," anthropologist Ian Tattersall of New York's American Museum of Natural History said in an email.
"Everything we know about evolutionary change suggests that genetic innovations are only likely to become fixed in small, isolated populations," he said. For example, Darwin's famous Galápagos finches each evolved from their mainland ancestor to fit a unique habitat on the isolated islands in the Pacific.
Natural selection, as outlined in On the Origin of Species, occurs when a genetic mutation—say, resulting in a spine suited to upright walking—is passed down through generations, because it affords some benefit. Eventually the mutation becomes the norm.
But if populations aren't isolated, crossbreeding makes it much less likely for potentially significant mutations to become established in the gene pool—and that's exactly where we are now, Tattersall said.
"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." Tattersall said. "Human beings are just going to have to learn to live with themselves as they are."
Steve Jones, a genetics professor at University College London, put forward a similar scenario during a recent lecture series marking the bicentenary of Darwin's birth and the 150th anniversary of On the Origin of Species at the University of Cambridge.
The human population will become more alike as races merge, he said, but "Darwin's machine has lost its power."
That's because natural selection—Darwin's "survival of the fittest" concept—is being sidelined in humans, according to Jones.
The fittest will no longer spearhead evolutionary change, because, thanks to medical advances, the weakest also live on and pass down their genes.
When On the Origin of Species was published in 1859, only about half of British children survived to 21. Today that number has swelled to 99 percent.
In developed countries, "the fact that everybody stays alive, at least until they're sexually mature, means ['survival of the fittest' has] got nothing to work with," Jones said. "That part of the Darwinian fuel has gone."
Humans Will Continue to Evolve
Other scientists see plenty of evidence that human evolution is far from over.
For instance, a study published last month in the journal Proceedings of the National Academy of Sciences suggested that women of the future could become shorter and stouter.
A team led by Yale University evolutionary biologist Stephen Stearns found that, due to ovulatory characteristics, shorter, slightly plumper women tend to have more children than their peers. These physical traits are passed on to their offspring, suggesting natural selection in humans is alive and well.
Geoffrey Miller, an evolutionary psychologist at the University of New Mexico, believes Darwinian evolution in humans is actually speeding up. He highlighted sexual selection through mate choice as one key driver.
"You still have powerful mate choice shaping mental traits particularly … traits that are needed to succeed economically and in raising kids," Miller said.
"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," he said.
"That intelligence results in higher earnings, social status, and sexual attractiveness."
Miller added that artificial selection using genetic technologies will likely accentuate these changes in the future.
"Parents could basically choose which sperm and egg get to meet up to produce a baby based on genetic information about which genes contribute to which physical and mental traits," he said.
"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," he added.
"But I think it's very likely the new genetic technologies will be widespread in their use, simply because that's more profitable. So I think there will actually be a leveling effect, where both the poor and the rich are going to be able to have the best kids they can genetically.
"You will probably see a rise in average physical attractiveness and health," he added. "You will probably get selection for physical traits that tend to be attractive in both males and females—things like height, muscularity, energy levels."
But "regular" natural selection will also continue to play a major role, Miller believes.
"What you're facing now is a global pathogen pool of viruses and bacteria that get spread around by air travel to every corner of the Earth, and that's going to increase," he said.
"We're going to get a lot more epidemics," Miller added. "That will increase the importance of the genetic immune system in human survival"—and result in a human species with stronger immune systems, he speculated.
Humans to Achieve Electronic Immortality
A philosophy known as transhumanism sees humans taking charge of their evolution and transcending their biological limitations via technology.
In essence, the old-fashioned evolution of On the Origin of Species may be beside the point: The future may belong to unnatural selection.
Nick Bostrom, director of the Future of Humanity Institute at the University of Oxford, said Darwinian evolution "is happening on a very slow time scale now relative to other things that are leading to changes in the human condition"—cloning, genetic enhancement, robotics, artificial intelligence, and nanotechnology, for starters.
Transhumanism raises a spectacular array of possibilities, from supersoldiers and new breeds of athletes to immortal beings who, having had their brains scanned atom by atom, transfer their minds to computers.
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, Bostrom imagines in his paper "The Transhumanist FAQ."
If that were to happen, a new type of evolution would emerge, Bostrom said.
"Evolutionary selection could occur in a population of uploads or artificial intelligence just as much as it could in a population of biological organisms," he told National Geographic News. "In fact, it might operate much faster there, because artificial intellects could reproduce much faster."
Whereas the current human generational cycle takes some 20 years, a digitalized individual could replicate themselves in seconds or minutes, Bostrom said.
Of course copying yourself isn't without complications, Bostrom acknowledges.
"Which one of them is you?" he writes. "Who owns your property? Who is married to your spouse?"
New Era of Evolution Awaits on Off-World Colonies?
"Some major new isolating mechanism" would be needed for a new human species to arise, according to John Hawks, an anthropologist at the University of Wisconsin-Madison.
Despite up to 30,000 years of partial isolation among populations in places such as Australia and Papua New Guinea, human speciation did not occur, he noted.
But if, in the far distant future, habitable planets beyond our solar system were colonized by Earth migrants, that could provide the necessary isolation for new human species to evolve.
"If we had spacefaring people who went on one-way voyages to distant stars, that might be enough to trigger speciation," Hawks said.
But, he added, "if you think about it, a small group of people went on a one-way voyage to [the Americas] 14,000 years ago, and then when new people [Europeans] showed up 500 years ago, they were still the same species."