• Did humans and Neanderthals 'do it'? Some experts doubt it

    M. Hofreiter / K. Fiusterweier

    Researchers say some Neanderthals may have had pale skin and red hair similar to that of some modern humans. Explaining the genetic similarities, however, can lead to a tangled tale.

    By Alan Boyle, Science Editor, NBC News


    One of the most titillating tales in the study of human origins — focusing on whether Neanderthals interbred with modern humans — has just gotten more tangled.

    Over the past couple of years, studies of Neanderthal DNA samples painstakingly extracted from ancient bones have suggested that contemporary non-Africans can trace up to 4 percent of their genetic code to our long-extinct Neanderthal cousins. The genomes of modern-day Africans, in contrast, have virtually nothing in common with the Neanderthals. Researchers assumed that the genetic contribution for the non-Africans was passed down through cross-species sex during the time that Neanderthals and Homo sapiens lived in close proximity in Europe, tens of thousands of years ago.

    However, there's another possibility: Maybe that common genetic code was passed down from the common ancestor of Homo sapiens and Homo neanderthalensis, hundreds of thousands of years ago in Africa. Today, researchers at the University of Cambridge reported in the Proceedings of the National Academy of Sciences that such a scenario provides a better fit for the genetic data. They say there's no need to assume that anatomically modern humans did the Neanderthal nasty, a process known more scientifically as hybridization. 

    "Our work shows clearly that the patterns currently seen in the Neanderthal genome are not exceptional, and are in line with our expectations of what we would see without hybridization," the lead researcher, Andrea Manica, said in a Cambridge news release. "So, if any hybridization happened — it's difficult to conclusively prove it never happened — then it would have been minimal and much less than what people are claiming now."

    Modeling population dynamics
    Manica and his colleagues set up a computer model for the last half-million years of population dynamics, with the assumption that there were two migrations from Africa. The first migration led to the settlement of Europe by the ancestors of the Neanderthals, hundreds of thousands of years ago. Then, around 300,000 to 350,000 years ago, the route from north Africa to Europe was cut off somehow. The European and north African populations showed gradual genetic divergence, but still retained a bit of common heritage from their mutual ancestors.

    When the second migration from Africa took place, around 60,000 to 70,000 years ago, the north Africans who dispersed to Europe and Asia would carry that extra bit of genetic similarity with them. But the Africans who lived farther south and stayed behind on the continent wouldn't have as much genetic kinship with the Europeans.

    The researchers found that their model did a fine job of accounting for the existing data without Neanderthal sex.

    So what do the researchers behind the earlier DNA studies say? That's where it gets really interesting: One study, published online in April in Molecular Biology and Evolution, contends that ancient population dynamics alone can't account for the genetic patterns seen in the DNA from Neanderthals and modern humans. Another study, posted on the arXiv preprint server and due for publication in PLOS Genetics, takes a closer look at a genetic pattern known as linkage disequilibrium — and concludes that modern humans and Neanderthals interbred somewhere between 37,000 and 86,000 years ago. Nature's Ewen Callaway delves into the details surrounding those claims and counterclaims.

    Either-or proposition?
    So, to recap: Some scientists say the population dynamics that were in effect hundreds of thousands of years ago can explain genetic similarities between populations, even if those populations never interbred. Others say the evidence is getting stronger that modern humans and Neanderthals really did mate when they met up in Europe, tens of thousands of years ago.

    University of Washington geneticist Joshua Akey says both sides just might be right.

    "To me, I don't think it's a case of either-or," he told me. "I think that both things can be going on."

    Akey and other researchers recently published a study in the journal Cell suggesting that a mysterious "Neanderthal sibling species" made a genetic contribution to the DNA of modern-day Africans. He said that interpreting whether genetic similarities come from a common ancestor (a process known as archaic population structure) or from more recent cross-species sex (a process that Akey calls introgression) is a tricky but essential task for those who study human origins.

    "Ultimately, it's important that we come to a consensus as to one process or the other, but I find them both to be interesting interpretations," Akey said. "Introgression is a sexier mechanism, but even if it turns out to be a case of archaic population structure, that still tells us something about our past that we didn't know before."

    So did modern humans do it with Neanderthals or not? And to what extent? Today, maybe it's a tangled tale, but that won't necessarily be the case forever. Akey said he was optimistic that researchers will be able to "tease apart" the different influences from the two processes within the next year or so.

    Update for 2:10 a.m. ET Aug. 14: In a follow-up email, Akey clarified his view on the "did they do it" question, leaning toward the affirmative:

    "Although I do think that both ancestral population structure and introgression are not mutually exclusive events, the recent papers from David Reich and Monty Slatkin show pretty compelling evidence that introgression of Neanderthal lineages into anatomically modern humans occurred. Thus, the real debate moving forward will be about the relative contributions of these two processes."

    More about ancient hominid sex:

    Cambridge's Anders Eriksson is Manica's co-author on the paper published by the Proceedings of the National Academy of Sciences, titled "Effect of Ancient Population Structure on the Degree of Polymorphism Shared Between Modern Human Populations and Ancient Hominins."

    Authors of the study in Molecular Biology and Evolution are Melinda Yang, Anna-Sapfo Malaspinas, Eric Durand and Montgomery Slatkin.

    Authors of the preprint destined for PLOS Genetics are Sriram Sankararaman, Nick Patterson, Heng Li, Svante Pääbo and David Reich.

    Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the dwarf planet and the search for new worlds.

  • Was there a fork in our family tree?

    msnbc.com

    How did pre-humans like australopiths, shown at left in this illustration, make the transition to early members of the genus Homo, shown at right? Perhaps it happened more than once.

    By Alan Boyle, Science Editor, NBC News

    The discoverer of the famous "Lucy" fossil says fresh findings suggest that more than one ancient species made the transition to more humanlike forms in different parts of Africa.

    Arizona State University paleoanthropologist Don Johanson shook up the scientific world in 1974 when he came across the traces of a 3.2 million-year-old skeleton in Ethiopia, a pre-human ancestor that came to be called Lucy. A similar shake-up may well be in the works due to the detailed analysis of another set of 1.977 million-year-old bones found in South Africa.

    In a series of studies published this week in the journal Science, researchers make a strong case that the bones, ascribed to a species called Australopithecus sediba, illustrate how the bodies of humanlike primates became more suited for upright walking, tool-making — and bigger brains.

    The international research team, led by the University of Witwatersrand's Lee Berger, says A. sediba is a good representative of the type of creature leading to the emergence of the genus Homo, which includes us Homo sapiens types as well as Neanderthals and a host of other now-extinct species.

    Courtesy of Donald Johanson

    Anthropologist Don Johanson holds a cast of the skull of Lucy, one of the world's best-known hominid fossils.

    But Johanson told me today that few of the reports about the latest findings touch on "the real crux of the matter." Even though A. sediba is a transitional form, with features of Australopithecus as well as Homo, he said there are other specimens of the genus Homo in eastern Africa that have been dated to roughly the same time. "There is a diversity of Homo already at 1.8 million years," he said.

    In fact, at least one of the fossil bones traditionally ascribed to Homo — an upper jaw from the same area of Ethiopia where Lucy was found — has been dated to an age of 2.3 million years, Johanson said. He sees that as a sign that some primates in east Africa had completed the transition to Homo while others in southern Africa were still in the midst of that transition.

    "Right after 3 million years toward the present, we see that there is a response in eastern and southern Africa which are on two different evolutionary trajectories," Johanson said. One trajectory led to grass-eaters such as Paranthropus robustus and Paranthropus boisei, and the other trajectory led to bigger-brained species such as Homo ergaster, Homo rudolfensis and Homo habilis. He said Homo habilis appears to have existed in east Africa at the same time that australopiths in southern Africa were becoming more Homo-like.

    Courtesy of Lee Berger

    Anthropologist Lee Berger holds the cranium of Australopithecus sediba.

    "For the very first time, we've found the roots of Homo in south Africa, but it's too late to be the roots of Homo in east Africa," Johanson said.

    During a teleconference, Berger said it can be difficult to tease out the relationships between the various species along the evolutionary path leading to modern humans.

    "We're dealing with a period between, say, 2.3 million years and 1.6 milion years where the entire remainder of the fossil record could fit into a small shoebox, as opposed to these very well-preserved skeletons," Berger said. But he insisted that A. sediba "may very well be as good a model or better than the Homo habilis one, which actually only has a larger brain to go with it." He pointed out that our knowledge about Homo habilis was based on "very fragmentary fossils."

    Darryl de Ruiter, an anthropologist at Texas A&M University who is part of Berger's team, said researchers considered whether A. sediba represented nothing more than an evolutionary dead end. "But as we pointed out, and as all these papers are demonstrating, in every aspect of the skeleton — cranium, teeth, jaw, mandibles, hand, pelvis, foot, everything that we look at — we see characteristics that align this species more closely with Homo than any other australopith," he said.

    When the discovery of the A. sediba fossils was announced last year, Johanson speculated that the species might be more appropriately considered a part of the genus Homo than the genus Australopithecus. "I've actually changed my view," Johanson said. Now he agrees with Berger's team that it's an australopith. And who knows? Anthropologists may well change their minds many times as more fossils come to light.

    In any case, Johanson said, this week's revelations are "very, very interesting."

    "It does show that there are probably different ways of being an upright walker, and there are different ways of arranging the anatomy," he said. "There isn't just one strict way."

    More about human evolution:

    Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding me to your Google+ circle. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for other worlds. 

  • Scientists say cavemen stayed put

    Darryl de Ruiter

    This specimen, known as SK 48, is one of the best examples of Paranthropus robustus from South Africa's Swartkrans Cave.

    By Alan Boyle, Science Editor, NBC News

    An analysis of fossil teeth from South Africa suggests that the males in pre-human societies stayed near the caves where they grew up, while the females migrated when it was time to mate.

    The researchers behind the analysis say their findings, published in this week's issue of the journal Nature, could shed light on the migratory behaviors that eventually gave rise to human societies.

    "This appears to be most similar to a chimpanzee social structure," said lead author Sandi Copeland, an anthropologist at the University of Colorado and Germany's Max Planck Institute for Evolutionary Anthropology. It's also consistent with the way many humans handle the process of moving out and settling down, she told journalists. "It's more often than not in modern societies that the woman is the one that leaves," she noted.

    But it's not the norm in the animal world. "In most primates and most mammals, it's usually the males who leave their home community," she said. Thus, the analysis could hint at one of the factors that made us human.

    Darryl de Ruiter

    The skull of "Mrs. Ples" is the most famous example of Australopithecus africanus from South Africa's Sterkfontein Cave.

    The research is the result of a "continuing effort that we have been making over the last five to 10 years," focusing on an area of South Africa that's rich with caves and pre-human fossils, said Oxford University archaeologist Julia Lee-Thorp, another one of the study's co-authors. She and her colleagues have been measuring the radioisotope distribution around the Sterkfontein and Swartkans cave sites, to come up with the chemical fingerprints for samples in that area.

    The point behind this exercise is to chart out the migration patterns for species that can no longer tell their own tales. By sampling the isotopes of strontium in tooth enamel, the team could determine which teeth belonged to animals that grew up in the cave region and absorbed strontium in the characteristic isotopic proportions from the foods they ate. That chemical signature remained intact in the tooth enamel throughout adulthood, even if the animals later roamed to a different area.

    Sandi Copeland

    Faint marks, left behind by a laser ablation procedure, can be seen along the right side of this Australopithecus africanus molar.

    The researchers carefully blasted 19 pre-human teeth with pinpoint laser beams, and then analyzed the results. Eight of the teeth came from the Sterkfontein Cave and were traced to a species known as Australopithecus africanus, dating back about 2.2 million years. The other 11 teeth came from the Swartkans Cave, and are attributed to Parathropus robustus, a species that lived about 1.8 million years ago. Australopithecus is thought to be a closer ancestor of modern humans than Paranthropus.

    The team expected to see a difference in the local vs. non-local distribution of the two species. They didn't.

    "As the numbers rolled off the mass spectrometer after each laser ablation, we were at first disappointed," Lee-Thorp said in an Oxford news release. "But we soon realized that we had found another prize — a difference between the males and females. It was totally unexpected."

    The researchers assumed that nine significantly larger teeth in their sample came from males, while the 10 significantly smaller teeth came from females. They chose these teeth for comparison precisely because of the size difference.

    Based on the strontium analysis, more than half of the female-sized teeth came from outside the cave region, while 90 percent of the male-sized teeth were traced to the caves. That finding led the researchers to conclude that the males were stay-at-home types, while the females were more likely to roam.

    Such a pattern is similar to that seen in chimpanzee societies, where males in a particular locale tend to stick together to defend their turf from interlopers. In order to guard against inbreeding, the younger females are likelier than the males to migrate for mating. Copeland said the situation is different for gorillas. In those societies, the dominant male gorilla rules over a harem that tends to stay put, while younger males usually have to go someplace else to find their own mates.

    "This study gets us closer to understanding the social structure of ancient hominds, since we now have a better idea about the dispersal patterns," Copeland said in a University of Colorado news release.

    The team's conclusions are based on less than two dozen teeth, divided up into species and sex categories. That raised questions about the small sample size. Texas A&M anthropologist Darryl de Ruiter, a study co-author, acknowledged that the team was "very constrained by the amount of material that we have available for destructive analysis." But he said that adding a few more teeth to the sample size may not have helped, because they had already selected the largest and the smallest teeth available to them.

    "Anything else we would have added would have been in the gray area," he told journalists. "It wouldn't have added any value to the discrimination between males and females."

    More about human origins:

    In addition to Copeland, Lee-Thorp and de Ruiter, co-authors of "Strontium Isotope Evidence for Landscape Use by Early Hominins" include Matt Sponheimer, Daryl Codron, Petrus J. le Roux, Vaughan Grimes and Michael P. Richards.

    You can connect with the Cosmic Log community by "liking" the log's Facebook page or following @b0yle on Twitter. Also, give a look to "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

  • Lucy's 'great-grandfather' found

    Dave Einsel / Getty Images file

    A sculptor's rendering shows how the 3.2-million-year-old hominid called Lucy might have looked in life. A more recently found fossil known as Kadanuumuu is from the same species, but 400,000 years older.

    Anthropologists say they have discovered the 3.6 million-year-old partial skeleton of a creature that came from the same species as Lucy, but was 400,000 years older and at least as good at walking upright. Their analysis suggests that upright walking, the trademark trait for humans and their extinct kin, goes back further in time than some might have assumed.

    This skeleton, described in the Proceedings of the National Academy of Sciences, has a much longer name than Lucy: It was dubbed Kadanuumuu, which means "big man" in Ethiopia's Afar language. Like the 3.3 million-year-old Lucy skeleton, Kadanuumuu was found in the East African country's Afar region, and shares the species name Australopithecus afarensis.

    Australopiths are fossil species that share some traits with chimpanzees - for instance, protruding faces and small brains - but share other traits with humans. Most importantly, their skeletons appear to have been built for upright walking. Arizona State University paleoanthropologist Donald Johanson, who discovered Lucy back in 1974, said the latest discovery adds to a "treasure trove" of hundreds of australopith fossils from East Africa.

    "It's like the El Dorado of paleoanthropology," he told me.

    Piecing together the evidence
    The first bone of Kadanuumuu's skeleton was found in 2005 in the Woranso-Mille area of the Afar region, about 30 miles north of where Lucy was discovered. Over the three years that followed, more than 30 additional bones were unearthed and pieced together for analysis.

    Hominid fossil

    CMNH / PNAS

    Elements of the partial australopith skeleton known as Kadanuumuu are arranged here anatomically.

    The head of the research team, Yohannes Haile-Selassie of the Cleveland Museum of Natural History, told me that Kadanuumuu's skeleton was clearly made for walking, based on measurements of bones including the limbs, clavicle and shoulder blade, the rib cage and the pelvis. In fact, its arrangement was better-suited for upright walking than Lucy's, even though it came from an earlier time in evolutionary history. The key measurement indicated that Kadanuumuu's lower limbs were more elongated than Lucy's - which would make walking easier.

    When Lucy was found, scientists thought her species was in the midst of a transition from tree-climbing to upright walking, but Kadanuumuu's larger skeleton suggests that the transition was already made hundreds of thousands of years earlier. (Haile-Selassie and his colleagues assume that Kadanuumuu was male, based on his size as well as the configuration of his pelvis.)

    "There is good grounds that advanced humanlike walking actually evolved long before people thought," Haile-Selassie said.

    So why did Lucy seem less-suited for upright walking? Haile-Selassie says it's because she was exceptionally small. Over the past 35 years, other specimens of Australopithecus afarensis have been found that suggested a body size larger than Lucy, and even larger than Kadanuumuu. "This individual is among the largest, but not the largest of all the specimens that we've found so far," Haile-Selassie said.

    Kadanuumuu is thought to have stood 5 to 5½ feet tall, while Lucy stood only 3½ feet tall. That's not unusual: Anthropologists have found that A. afarensis exhibited significant size differences between the male and the female of the species, a quality known as sexual dimorphism. The diminutive stature of Lucy, which is still the most complete australopith skeleton found to date, may have initially led some scientists down the wrong path, Haile-Selassie said. "Most of the misinterpretations were largely based on the size of Lucy and her sex," he told me.

    Findings fit in with ancient footprints
    If the conclusions made by Haile-Selassie and his colleagues are correct, the saga of how we became human is much more ancient than some might have thought. But in fact, the conclusions are consistent with another famous find, the 1976 discovery of the Laetoli footprints in Tanzania. Those prints, which were preserved in volcanic ash 3.6 million years ago, led scientists to suggest that upright walking was mastered well before Lucy's time. "What we have now is the skeletal evidence to complement those footprints," Haile-Selassie said.

    Johanson agreed. "This supports much of what we've known before" about the ability of australopiths to walk upright, he told me. He's not fully convinced, however, that Kadanuumuu was significantly better-built for walking than Lucy was. "I'm not quite sure they really have enough to say that the lower limb is elongated," he said.

    All this could lead anthropologists to look further back for the origins of upright walking. Perhaps Australopithecus anamensis, which lived in East Africa between 4.2 million and 3.9 million years ago, was the species that picked up the trick. Perhaps it all started with Ardipithecus ramidus, which is thought to have split its time between the trees and the ground in Ethiopia 4.4 million years ago (though there's some controversy over that claim).

    That doesn't mean Australopithecus afarensis is out of the spotlight when it comes to studying human origins. Johanson said Lucy and her kin provide an "important reference for assessing other hominid species," in large part because so many specimens have been found over such a wide span of evolutionary time. Going forward, paleoanthropologists may well turn to Lucy, Kadanuumuu and other members of the species to unravel the deeper secrets of ancient human development.

    "You can begin to look at the minutiae of microevolution over time," Johanson said, "which is where we're heading."

    More on the human origin story:

    In addition to Haile-Selassie, the authors of "An Early Australopithecus Afarensis Postcranium From Woranso-Mille, Ethiopia" include Bruce M. Latimer, Mulugeta Alene, Alan L. Deino, Luis Gilbert, Stephanie M. Melillo, Beverly Z. Saylor, Gary R. Scott and C. Owen Lovejoy.

    This report was last updated at 9 p.m. ET.

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