DNA Stunner

Scientists have recovered the oldest known DNA from a member of the human evolutionary family. This find raises surprising questions about relationships among far-flung populations of ancient hominids.

A nearly complete sample of mitochondrial DNA was extracted from a 400,000-year-old leg bone previously found in a cave in northern Spain. The DNA shows an unexpected hereditary link to the Denisovans, Neandertals’ genetic cousins that lived in East Asia at least 44,000 years ago, say paleogeneticist Matthias Meyer of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and his colleagues. ¹

The fact that the Sima de los Huesos mtDNA shares a common ancestor with Denisovan rather than Neanderthal mtDNAs is unexpected in light of the fact that the Sima de los Huesos fossils carry Neanderthal-derived features (for example, in their dental, mandibular, midfacial, supraorbital and occipital morphology). Denisovans were identified in 2010 based on DNA sequences retrieved from a manual phalanx [finger] and a molar [tooth] found in southern Siberia. ³

Furthermore, although almost no morphological [shape] information is available for Denisovans, a molar that carries Denisovan DNA is of exceptionally large size and does not exhibit the cusp reduction seen in the Sima de los Huesos hominins. ⁴

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. ⁵

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. ⁶

If the Sima hominids’ ancestors mated with members of another hominid species — possibly Homo erectus or an as-yet-undiscovered population —mitochondrial DNA variants could have entered the Sima DNA and later reached the Denisovans via interbreeding with the same species, Meyer speculates.

Another possibility is that Denisovan ancestors occupied a vast expanse of Asia and Europe before the Sima population evolved, says paleogeneticist Carles Lalueza-Fox of the Institute of Evolutionary Biology in Barcelona. Hominid fossils found in two caves near Sima de los Huesos, dating to between 1.3 million and 800,000 years ago, may represent descendants of that intercontinental population, Lalueza-Fox suggests. Sima hominids thus could have received genetic contributions from those groups that partly matched DNA separately inherited by the Denisovans far to the east.

If so, Neandertals probably originated as a small, isolated European population around 250,000 years ago, Lalueza-Fox proposes. ⁷