Year in Review: New discoveries reshape debate over human ancestry

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Human evolution appears poised for a scientific makeover, as unexpected and provocative findings have raised new questions this year about two poorly understood periods leading to the emergence of Homo sapiens.

The biggest conundrum comes courtesy of the oldest known DNA sample from a member of the human evolutionary family — a 400,000-year-old leg bone previously found in a cave in northern Spain. A nearly complete sample of maternally inherited mitochondrial DNA linked the bone to the Denisovans, mysterious genetic cousins of Neandertals who lived in Siberia at least 44,000 years ago (SN: 12/28/13, p. 8).

Paleogeneticist Matthias Meyer of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and colleagues wrung DNA from a fossil found at Sima de los Huesos, or pit of bones, a site where the remains of at least 28 Stone Age individuals have been excavated. Many researchers classify these finds as Homo heidelbergensis, a species regarded as an ancestor of Neandertals and perhaps also of Homo sapiens.

It would have been no shock to find mitochondrial DNA links between the Sima fossil and Neandertals, a species that has yielded partial samples of mitochondrial DNA from as early as 100,000 years ago. Now, however, scientists must try to figure out how a genetic connection formed between H. heidelbergensis in western Europe and presumably later-evolving Denisovans in Asia.

Several evolutionary scenarios have been suggested. Ancestors of Sima hominids in Europe and of Denisovans in Asia could have mated with members of another humanlike species, such as Homo erectus, which transmitted common mitochondrial DNA segments to both populations. Or ancient Denisovan ancestors may have occupied parts of Europe as well as Asia and passed on the same genetic traits to the Sima population and eventually to the Denisovans.

Meyer’s team will now try to wrest biparentally inherited nuclear DNA from a Sima fossil in hopes of getting a better look at how evolution played out among populations that set the stage for H. sapiens’ appearance around 200,000 years ago.

Questions also came up this year about the origins of the Homo genus, a group of upright-walking species that led to people today. New analyses of recently unearthed skeletons highlighted opposing views about how the Homo line evolved between 3 million and 2 million years ago.

One controversial idea, championed by paleoanthropologist Lee Berger of the University of the Witwatersrand in Johannesburg, South Africa, holds that an odd-looking member of the human evolutionary family living in southern Africa 2 million years ago was the ancestor of the first Homo species. Most researchers suspect that the Homo genus emerged in eastern Africa.

Berger’s team studied two partial skeletons excavated since 2008 in a cave at South Africa’s Malapa site. These finds come from a previously unknown species, Australopithecus sediba, that walked on two legs awkwardly and had a small brain (SN: 8/10/13, p. 26). The nearly 2-million-year-old Malapa fossils share some key traits with the Homo genus, including round brain cases and long, inward-curving lower backs. A. sediba could have originated hundreds of thousands of years earlier and evolved into H. erectus, a species that also appeared in Africa around 2 million years ago, Berger argued.

But discoverers of a 1.8-million-year-old H. erectus skull in western Asia said that the fossil and four other skulls previously found at the same site sink Berger’s southern scenario. The Asian fossils display enough variation in early Homo fossils from eastern Africa — including one from 2.3 million years ago — also belonged to H. erectus (SN: 11/16/13, p. 6). David Lordkipanidze, a paleoanthropologist of the Georgian National Museum in Tbilisi, concluded that H. erectus probably evolved across parts of Asia and Africa, while A. sediba eventually went extinct.

From Spain to Siberia and from the South Caucasus to South Africa, scientific understanding of human ancestry seems poised for some evolution of its own.

See all top science stories of 2013