Fossil teeth push the human-Neandertal split back to about 1 million years ago

People and Neandertals separated from a common ancestor more than 800,000 years ago — much earlier than many researchers had thought.

That conclusion, published online May 15 in Science Advances, stems from an analysis of early fossilized Neandertal teeth found at a Spanish site called Sima de los Huesos. During hominid evolution, tooth crowns changed in size and shape at a steady rate, says Aida Gómez-Robles, a paleoanthropologist at University College London. The Neandertal teeth, which date to around 430,000 years ago, could have evolved their distinctive shapes at a pace typical of other hominids only if Neandertals originated between 800,000 and 1.2 million years ago, she finds. 

Gómez-Robles’ study indicates that, if a common ancestor of present-day humans and Neandertals existed after around 1 million years ago, “there wasn’t enough time for Neandertal teeth to change at the rate [teeth] do in other parts of the human family tree” in order to end up looking like the Spanish finds, says paleoanthropologist Bernard Wood of George Washington University in Washington, D.C.

Many researchers have presumed that a species dubbed Homo heidelbergensis, thought to have inhabited Africa and Europe, originated around 700,000 years ago and gave rise to an ancestor of both Neandertals and Homo sapiens by roughly 400,000 years ago. Genetic evidence that Sima de los Huesos fossils came from Neandertals raised suspicions that a common ancestor with H. sapiens existed well before that (SN Online: 3/14/16). Recent Neandertal DNA studies place that common ancestor at between 550,000 and 765,000 years old. But those results rest on contested estimates of how fast and how consistently genetic changes accumulated over time.

With that molecular debate in mind, Gómez-Robles calculated the rate at which eight ancient hominid species evolved changes in tooth shape. That enabled her to gauge how long it must have taken for Sima de los Huesos teeth to evolve after Neandertals diverged from a common ancestor with H. sapiens.

Gómez-Robles used two possible evolutionary trees for the eight hominid species to estimate dental evolution rates. Aside from the Spanish Neandertals and Stone Age H. sapiens, teeth in her study came from African hominids dating to as early as 3.2 million years ago.

Moving back the date of an evolutionary split between Neandertals and H. sapiens appears reasonable based on the new data, says paleoanthropologist Aurélien Mounier of Musée de l’Homme in Paris. The timing of that split could still change, though, if further research modifies the Spanish fossils’ age, he says.

Other Spanish hominid teeth dating to nearly 800,000 years ago display some Neandertal features, supporting the new study’s conclusions, says New York University paleoanthropologist Shara Bailey. But it’s unclear if Gómez-Robles’ contention that hominid teeth evolved at a steady rate will hold true, Bailey says.