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Two studies of ancient humans have shed new light on the last common ancestor we share with Neanderthals. An extinct species that was once in the frame now looks unlikely to be the one. Another now seems more plausible, but it may only be related to the ancestor.

“My guess is we haven’t found the common ancestor yet,” says Chris Stringer of the Natural History Museum in London. However, the new findings clarify what the common ancestor may have looked like.

Broken Hill

Stringer’s team studied a skull called Kabwe 1, which was discovered in 1921 by miners at Broken Hill in what is now Zambia. “It was the first important [hominin] fossil found in Africa,” says Stringer. It probably belonged to a young male and had a primitive-looking face with “huge brow ridges over the eyes”.

Many anthropologists place Kabwe 1 in Homo heidelbergensis, which ranged across Africa and Europe between about 700,000 and 300,000 years ago. It has long been a candidate for the common ancestor of three later groups: modern humans (Homo sapiens), the Neanderthals of Europe and west Asia, and the Denisovans of east Asia.

However, until now the Kabwe skull’s age has been a mystery. The normal approach is to date the surrounding sediments, says Rainer Grün of Griffith University in Australia. But the skull was found by accident and the site quarried, so researchers have no sediments to test. The general assumption has been that the skull is about 500,000 years old, but it has not been possible to assess that idea properly.

“The only thing we could do is to analyse the skull itself,” says Grün. This is only now possible. Older methods would have required drilling into the skull, causing “unacceptable” damage. Instead, the team used lasers to remove fragments a quarter of a millimetre thick.

Analyses of these fragments indicate Kabwe 1 is about 299,000 years old.

“It’s pretty incredible that the new techniques they’re coming up with allow us to go back and directly date the fossils themselves,” says Shara Bailey of New York University.

The age of Kabwe 1 means the skull probably didn’t belong to an ancestor of humans, Neanderthals and Denisovans, says Stringer. Genetic evidence suggests the last common ancestor lived about 600,000 years ago, so Kabwe 1 is too recent. There is also evidence that modern humans were present in northern Africa 300,000 years ago, about the same time Kabwe 1 was alive.

“We reassess it as a separate line of evolution, but one which probably coexisted with the evolution of Homo sapiens,” says Stringer.

“It shows there were tonnes of different hominins running around in Africa 300,000 years ago,” says Lauren Schroeder at the University of Toronto.

“We are changing our paradigm for the origins of Homo sapiens,” says Bailey. Instead of a simple progression from one species to the next, many groups coexisted and sometimes interbred. “This was a process that probably happened across Africa,” she says.

This means H. heidelbergensis did not simply evolve into modern humans, Neanderthals and Denisovans – although it is theoretically possible that one small H. heidelbergensis population in existence about 700,000 years ago could have been the common ancestor of the later humans. But there is an alternative.

Spanish contender

A better candidate for the common ancestor is Homo antecessor, says José María Bermúdez de Castro of the National Centre for Research on Human Evolution in Spain. These hominins lived in northern Spain between 1.2 million and 800,000 years ago.

“H. antecessor shows a unique combination of dental and skeletal features,” says Bermúdez de Castro. Its face was quite modern: more like ours than like that of H. heidelbergensis.

His colleagues have now extracted seven proteins from an H. antecessor tooth from about 860,000 years ago. This represents a major breakthrough. “We are able to reliably retrieve ancient human protein sequences over the past 2 million years,” says Frido Welker at the University of Copenhagen in Denmark.

By comparing the H. antecessor proteins with those of other hominins, the team has found that the species was closely related to the last common ancestors of humans, Neanderthals and Denisovans. However, the researchers cannot tell if it actually was the ancestor. “It is too early to conclude this confidently,” says Enrico Cappellini at the University of Copenhagen.

Ancient sister

“It’s actually more likely, from the palaeoproteomics evidence, that H. antecessor is a sister [group] to Neanderthals, Denisovans and modern humans,” says Schroeder. “That means that we still don’t know who the last common ancestor of all these groups is.”

There is also an issue of timing, says Stringer. “If that ancestor lived 600,000 years ago, that’s at least 200,000 years after H. antecessor,” he says.

However, the ancestor’s age remains uncertain. A 2019 study suggested the , because Neanderthal teeth differ from ours, and teeth can only evolve so fast. If so, H. antecessor is still a possibility.

Either way, H. antecessor is probably a better guide to what the ancestor looked like than H. heidelbergensis, says Stringer. “It’s got to be something with a face more like us and H. antecessor.” In other words, the modern-looking face of H. antecessor is actually ancient and our species has retained it, whereas Neanderthals are the ones whose faces changed more during their evolution.

Journal references: Nature, and