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New timelines and questions raised about human evolution due to recent findings

Discovery of a Neanderthal temporal bone trait in an ancient skull raises new questions about human evolution. Modern humans emerged from a complex 'labyrinth of biology and peoples,' findings suggest, according to research, "Temporal labyrinths of eastern Eurasian Pleistocene humans," published online July 7, 2014 in the Proceedings of the National Academy of Sciences. Re-examination of a circa 100,000-year-old archaic early human skull found 35 years ago in Northern China has revealed the surprising presence of an inner-ear formation long thought to occur only in Neanderthals.

New timelines and questions raised about human evolution due to recent findings.
Photo by Win McNamee/Getty Images

"The discovery places into question a whole suite of scenarios of later Pleistocene human population dispersals and interconnections based on tracing isolated anatomical or genetic features in fragmentary fossils," said study co-author Erik Trinkaus, PhD, according to a July 7, 2014 news release, "Discovery of Neandertal trait in ancient skull raises new questions about human evolution." Trinkaus is a physical anthropology professor at Washington University in St. Louis. "It suggests, instead, that the later phases of human evolution were more of a labyrinth of biology and peoples than simple lines on maps would suggest."

Interbreeding and gene transfer between Neanderthals and modern human ancestors

The study is based on recent micro-CT scans revealing the interior configuration of a temporal bone in a fossilized human skull found during 1970s excavations at the Xujiayao site in China's Nihewan Basin. Trinkaus, the Mary Tileston Hemenway Professor in Arts & Sciences, is a leading authority on early human evolution and among the first to offer compelling evidence for interbreeding and gene transfer between Neanderthals and modern human ancestors.

"We were completely surprised," Trinkaus said, according to the news release. "We fully expected the scan to reveal a temporal labyrinth that looked much like a modern human one, but what we saw was clearly typical of a Neanderthal. This discovery places into question whether this arrangement of the semicircular canals is truly unique to the Neanderthals." His co-authors on this study are Xiu-Jie Wu, Wu Liu and Song Xing of the Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, and Isabelle Crevecoeur of PACEA, Université de Bordeaux.

Often well-preserved in mammal skull fossils, the semicircular canals are remnants of a fluid-filled sensing system that helps humans maintain balance when they change their spatial orientations, such as when running, bending over or turning the head from side-to-side

Since the mid-1990s, when early CT-scan research confirmed its existence, the presence of a particular arrangement of the semicircular canals in the temporal labyrinth has been considered enough to securely identify fossilized skull fragments as being from a Neanderthal. This pattern is present in almost all of the known Neanderthal labyrinths. It has been widely used as a marker to set them apart from both earlier and modern humans.

The skull at the center of this study, known as Xujiayao 15, was found along with an assortment of other human teeth and bone fragments, all of which seemed to have characteristics typical of an early non-Neanderthal form of late archaic humans. Trinkaus, who has studied Neanderthal and early human fossils from around the globe, said, according to the news release, that this discovery only adds to the rich confusion of theories that attempt to explain human origins, migrations patterns and possible interbreedings.

Possible interbreeding or more widespread labyrinth genes?

While it's tempting to use the finding of a Neanderthal-shaped labyrinth in an otherwise distinctly "non-Neanderthal" sample as evidence of population contact (gene flow) between central and western Eurasian Neanderthals and eastern archaic humans in China, Trinkaus and colleagues argue that broader implications of the Xujiayao discovery remain unclear.

"The study of human evolution has always been messy, and these findings just make it all the messier," Trinkaus said, according to the news release. "It shows that human populations in the real world don't act in nice simple patterns.

"Eastern Asia and Western Europe are a long way apart, and these migration patterns took thousands of years to play out," he said, according to the news release. "This study shows that you can't rely on one anatomical feature or one piece of DNA as the basis for sweeping assumptions about the migrations of hominid species from one place to another."

The assessment of the paleobiology and morphological affinities of the Neanderthals and other Late Pleistocene archaic humans is central to resolving issues regarding the emergence and establishment of modern human morphology and diversity, says the study's abstract. One feature, which has been used as a distinctive Neanderthal feature in the context of the research paper, is the apparently derived shape of their temporal labyrinths (especially semicircular canals). The bones (semicircular canals) of the Neanderthal ear were different in shape from modern Homo Sapiens.

What the research team analyzed were prehistoric remains of East Asian labyrinths. The skulls analyzed document the “Neanderthal” pattern in the Xujiayao 15 temporal bone, although none of the Xujiayao human remains exhibits other distinctly Neandertal features, the study's abstract explains. It therefore raises questions regarding possible biological correlates of labyrinthine morphology, distinctive Neanderthal features, and the nature of late archaic human variation across Eurasia. Now the question remains, do the findings represent admixture between Homo Sapiens and Neanderthals at that time? Or did prehistoric people of that time have more variation in their temporal bones?

In another study, Smithsonian scientists revised the timeline of human origins

A Smithsonian scientist and collaborators revised the timeline of human origins. A new synthesis of research links the changing environment with Homo's evolutionary adaptability. Many traits unique to humans were long thought to have originated in the genus Homo between 2.4 and 1.8 million years ago in Africa. Although scientists have recognized these characteristics for decades, they are reconsidering the true evolutionary factors that drove them. The new research is "Evolution of early Homo: An integrated biological perspective," published online July 4, 2014 in Science.

A large brain, long legs, the ability to craft tools and prolonged maturation periods were all thought to have evolved together at the start of the Homo lineage as African grasslands expanded and Earth's climate became cooler and drier. However, new climate and fossil evidence analyzed by a team of researchers, including Smithsonian paleoanthropologist Richard Potts, Susan Antón, professor of anthropology at New York University, and Leslie Aiello, president of the Wenner-Gren Foundation for Anthropological Research, suggests that these traits did not arise as a single package. Rather, several key ingredients once thought to define Homo evolved in earlier Australopithecus ancestors between 3 and 4 million years ago, while others emerged significantly later.

Synthesis of data pointed to the ability of early humans to adjust to changing conditions in order to survive

The team's research takes an innovative approach to integrating paleoclimate data, new fossils and understandings of the genus Homo, archaeological remains and biological studies of a wide range of mammals (including humans). The synthesis of these data led the team to conclude that the ability of early humans to adjust to changing conditions ultimately enabled the earliest species of Homo to vary, survive and begin spreading from Africa to Eurasia 1.85 million years ago.

Potts developed a new climate framework for East African human evolution that depicts most of the era from 2.5 million to 1.5 million years ago as a time of strong climate instability and shifting intensity of annual wet and dry seasons. This framework, which is based on Earth's astronomical cycles, provides the basis for some of the paper's key findings, and it suggests that multiple coexisting species of Homo that overlapped geographically emerged in highly changing environments.

"Unstable climate conditions favored the evolution of the roots of human flexibility in our ancestors," said Potts, according to the July 3, 2014 news release, "Smithsonian scientist and collaborators revise timeline of human origins." Potts is curator of anthropology and director of the Human Origins Program at the Smithsonian's National Museum of Natural History. "The narrative of human evolution that arises from our analyses stresses the importance of adaptability to changing environments, rather than adaptation to any one environment, in the early success of the genus Homo."

The team reviewed the entire body of fossil evidence relevant to the origin of Homo to better understand how the human genus evolved. For example, five skulls about 1.8 million years old from the site of Dmanisi, Republic of Georgia, show variations in traits typically seen in African H. erectus but differ from defining traits of other species of early Homo known only in Africa. Recently discovered skeletons of Australopithecus sediba (about 1.98 million years old) from Malapa, South Africa, also include some Homo-like features in its teeth and hands, while displaying unique, non-Homo traits in its skull and feet. Comparison of these fossils with the rich fossil record of East Africa indicates that the early diversification of the genus Homo was a period of morphological experimentation. Multiple species of Homo lived concurrently.

"We can tell the species apart based on differences in the shape of their skulls, especially their face and jaws, but not on the basis of size," said Antón. "The differences in their skulls suggest early Homo divvied up the environment, each utilizing a slightly different strategy to survive."

Even though all of the Homo species had overlapping body, brain and tooth sizes, they also had larger brains and bodies than their likely ancestors, Australopithecus. According to the study, these differences and similarities show that the human package of traits evolved separately and at different times in the past rather than all together.

In addition to studying climate and fossil data, the team also reviewed evidence from ancient stone tools, isotopes found in teeth and cut marks found on animal bones in East Africa.

"Taken together, these data suggest that species of early Homo were more flexible in their dietary choices than other species," said Aiello, according to the news release. "Their flexible diet—probably containing meat—was aided by stone tool-assisted foraging that allowed our ancestors to exploit a range of resources."

The team concluded that this flexibility likely enhanced the ability of human ancestors to successfully adapt to unstable environments and disperse from Africa. This flexibility continues to be a hallmark of human biology today, and one that ultimately underpins the ability to occupy diverse habitats throughout the world. Future research on new fossil and archaeological finds will need to focus on identifying specific adaptive features that originated with early Homo, which will yield a deeper understanding of human evolution.

On another note, you may wish to check out the July 4, 2014 article, "European neuroscientists revolt against the E.U.'s Human Brain Project." Or see, "Early Start for Human Art? Ochre May Revise Timeline."