We’ve all seen those textbook illustrations of human evolution: a straight line of ape-like hominids progressing into less-hairy and more straight-backed versions until they turn into us - Modern Man.
Great, you can now throw all that away. It turns out that our evolution was less of a linear march and more of a messy pile-up of ancestral species, constantly branching out and going extinct, interacting with each other and, of course, having sex.
That is the overarching message of the many discoveries made this year that have changed the way we understand ourselves and our very origin.
It seems now that Homo sapiens has been around for much longer than we thought, and that our ancestors were also clearly leaving Africa tens or even hundreds of thousands of years before the vaunted great exodus of 60,000 years ago. They were intermixing with other hominid species and, it seems, not all those "early waves" of people went extinct.
In 2010, Israeli archeologists announced the discovery of eight teeth in Qesem, a prehistoric site just a few kilometers east of Tel Aviv.
The teeth found in the cave were puzzling. They shared features common to both Neanderthals and Homo sapiens, i.e., modern humans. But the remains were impossibly old.
Dated to between 200,000 and 400,000 years ago, the teeth were from an age when Neanderthals and modern humans were not supposed to have existed yet, at least beyond their points of origin in Europe and Africa respectively. At both ends of the possible range, neither Neanderthal nor modern humans were supposed to be in the Middle East yet.
With no additional human remains emerging from Qesem, a handful of teeth was hardly enough evidence to seriously rewrite the story of human evolution, and researchers filed away the strange teeth and forbore to reach conclusions.
But this year, all that changed. This year, a series of discoveries is forcing many researches to rethink what we know about our ancestors, pushing back the clock on the evolution of Homo sapiens and changing the picture of our emergence from Africa and early interaction with other hominids.
Until recently most scholars believed that anatomically modern humans first appeared just under 200,000 years ago in East Africa, based on fossils of that age found at Omo Kibish in Ethiopia. Also, genetic analyses done since the 1990s indicated that all humans alive today descended from a single group of sapiens that began moving out of Africa some 60,000 to 70,000 years ago.
According to the genetic research, these pioneering humans reached the Middle East between 60,000 to 50,000 years ago, where they likely met and interbred with the dwindling Neanderthals, leaving all non-African populations with a small percentage of Neanderthal DNA.
While the last part of the story still stands, the idea of a relatively quick and late evolution followed by a single exodus from Africa no longer squares up with the most recent evidence, experts interviewed by Haaretz say.
“Until recently we thought that modern humans appeared something like 200,000 years ago, maximum 250,000 years ago,” says Israel Hershkovitz, a physical anthropologist from Tel Aviv University. “It now seems that our ancestors appeared much earlier. How earlier is an interesting question.”
In the early 1960s, a hominin skull and other bones with primitive features had been found in a cave in Jebel Irhoud, Morocco. At the time, the researchers dated the remains to 40,000 years ago and identified them as Neanderthal – even though no remains from this species had been found in Africa, before or since.
In June, following new digs at the site and tests using more advanced techniques, the remains were reanalyzed, with a result that shocked the scientific community. This was no Neanderthal. The remains belonged to a Homo sapiens and were at least 300,000 years old, far predating the remains known from eastern Africa. The Jebel Irhoud bones push back the emergence of our species by at least 100,000 years.
Further confounding researchers, a skull closely resembling the Moroccan remains and dating to 260,000 years ago was found in the Dali region of China – 10,000 kilometers from Jebel Irhoud. "Dali Man" has its detractors, with some researchers – including Dr. Jean-Jacques Hublin, director of the Human Evolution department at the Max Planck Institute for Evolutionary Anthropology – suggesting it could be "a representative of the Denisovans," a primitive species closely related to the Neanderthals that is known to have lived in Asia.
Still, with all these early putative sapiens remains turning up across the world, those surprisingly modern-looking teeth discovered at Qesem cave are starting to look like less of a red herring.
Researchers have also deduced that the hominids who lived at the site in central Israel used standardized techniques, which means they were passed on from generation to generation, to butcher animals and make flint tools. If so, it further suggests that the dwellers in the Levant hundreds of thousands of years ago had a relatively advanced culture and were capable of using communication to transmit knowledge to children.
But the most important evidence in support of an early evolution of modern humans is emerging not from archaeological digs but from the labs of geneticists.
Geneticists can estimate when a species arose by its so-called "molecular clock," using the mutation rate of DNA to estimate how far back in time a certain species split off from its ancestral genetic line. Again, the method is controversial – not even geneticists agree how "fast" these "molecular clocks" move. That said, one study, published in September in Science, sought to evaluate the theory that modern man arose in South Africa, not in the continent's north. The paper compared the DNA of 2,000-year-old hunter gatherers in South Africa to more recent remains from 300 to 500 years ago. The molecular clock estimates suggested that that the earliest modern humans diverged from their last common ancestor between 350,000 to 260,000 years ago.
But the real scoop on sapiens DNA came from a source that wasn’t even supposedly connected to sapiens: Neanderthals.
O brother Neanderthal, where art thou?
The next startling discovery was to come out of a leg bone found in Germany, which was part of the stomping ground of Neanderthals. The Neanderthals were a member of the Homo group, characterized by large brains and advanced tool use.
Generally, Neanderthals (and Denisovans) are believed to have arisen from some common ancestor that reached Eurasia. That is still not disproved. Also, we are reasonably confident now that all three of these hominins – us, Neanderthals and Denisovans – mixed. The question is where and when.
Genetic analysis of a 120,000-year-old femur of a Neanderthal from southwestern Germany showed it to be much more similar to modern humans than early Neanderthals, based on sequencing of mitochondrial DNA (which passes on exclusively from the mother).
Now, Neanderthals and modern humans were supposed to have branched off more than 700,000 years ago from their last common ancestor and then to have lived in relative isolation, Neanderthals in Eurasia and Homo sapiens in Africa, until that famous exodus from Africa 60,000 years ago.
Evidently that is not so, says Cosimo Posth, a paleogeneticist at the Max Planck Institute for the Science of Human History in Jena, Germany, and lead of author of the study published earlier this year in Nature Communications.
The mitochondrial DNA analysis shows that at a point between 220,000 and 460,000 years ago a “modern-like” population from Africa mixed with at least part of the Neanderthal lineage, Posth says.
That is a vast range in time, but the whole range predates the time when modern humans were supposed to have evolved, leave Africa and meet Neanderthals to have sex with.
“This is an indication that modern humans have a much earlier origin, and that contact between these archaic groups happened much more often than we thought,” Posth told Haaretz.
Prehistoric alphabet soup
It’s too early to tell whether this “introgression” – as scientists unromantically call it – came from our direct ancestors, or a parallel lineage of modern humans, says Posth.
Since no Neanderthal remains have been found (so far) in Africa, one possible scenario could be that a group of early humans like those living around Jebel Irhoud managed to cross the Straits of Gibraltar.
Another, perhaps more probable scenario, is that the admixture happened in Israel or the Middle East, where there is evidence that Homo sapiens and Neanderthals coexisted for thousands of years.
“There was a constant in and out of Neanderthals and Homo sapiens in the Levant,” Tel Aviv's Israel Hershkovitz notes. “We already saw that in Qesem, we just couldn’t interpret it at the time. We said [the teeth] had Neanderthal and Homo sapiens characteristics, and everyone said it couldn’t be.”
Today, the Israeli anthropologist adds, “We know that the Neanderthals are much more important to our evolution than we thought and our relationship with them was not a one-time, late episode: we have a shared history that goes back maybe half a million years.”
Researchers have long had reason to suspect that sapiens was running around the world before the purported exodus from Africa. Modern human remains found nearly a century ago in the Qafzeh and Skhul caves in the Galilee have been dated to around 100,000 years ago – well before the big out-of-Africa event.
The Qafzeh and Skhul skulls could be easily dismissed as small foray by a group of archaic sapiens who quickly went extinct. Besides, many researchers consider that from an anthropological and evolutionary point of view Israel is part of Africa.
Now, with sapiens remains and genetic code cropping up across the globe, it seems the story was quite different.
“Those archaic homo sapiens are leaving Africa probably even before Qafzeh-Skhul and possibly getting all the way to China and all over the place and they are interacting with other archaic homo species like Neanderthals or Denisovans,” says Daniel Adler, a professor of anthropology at the University of Connecticut. “There is a whole alphabet soup of hominins moving throughout Eurasia in the late middle Pleistocene,” a period ranging from 781,000 to 126,000 years ago.
This constant mixture may also help explain why the flint tools and other material remains from that period tend to be very similar across the world, making it difficult for archaeologists to link a specific hominin with a certain site unless they are lucky enough to find human fossils.
"In the absence of fossil material, when we find, say, a middle Paleolithic assemblage in Eurasia we assume it’s Neanderthal, but we have no way of proving that,” Adler says, adding that this phenomenon may have encouraged researchers to neatly pigeonhole their discoveries rather than recognize and embrace the complexity.
The pattern that is emerging now is that new populations of early Homo sapiens were constantly emerging from Africa, "leaking" out of the continent, interbreeding and coexisting with other hominids, he says.
Extinct no more
While some of these groups may have gone extinct, others may have survived until that last big exodus from Africa - and some of us are apparently carrying around these other groups' DNA in us.
In fact, some finds suggest that those early migrations may have reached as far as Australia around 65,000 years ago, which is before our ancestors had even been thought to leave Africa.
If these archaic humans usually settled amongst and coexisted with other hominids, how did we end up being the only surviving species of homo mere thousands of years after the last great exodus from Africa 60,000 to 70,000 years ago?
Some researchers suggest everything from environmental factors to epidemics or even open warfare as factors in the disappearance of Neanderthals and other hominids. Others believe modern sapiens had a cognitive or evolutionary advantage over his fellow hominids.
The reality may be more mundane. During the Ice Age, Africa could probably sustain much larger populations of people than the frozen steppes of Eurasia. Any local group in Eurasia would have been gradually absorbed and supplanted by the newcomers thanks to simple demographic forces.
Especially for hunter-gatherers, there is strength in numbers, Adler notes.
“We probably had larger populations and those populations were socially better integrated,” he says. “Larger social networks, knowing more people, means more people to fall back onto in lean times, exchange information with about the unknown and reduce risks.”
Whether or not Homo sapiens had improved language or better tools, which scientists argue about to this day, support for this by-the-numbers approach also comes from a recent study by Stanford University researchers, which used computer modeling to argue that the constant migratory trickle from Africa would be enough to spell doom for the Neanderthals and leave us as the single surviving human species on Earth.