This 300,000-year-old skull may be from an African ‘ghost’ population

A mysterious but well-preserved hominid skull found nearly a century ago comes from a population that lived in Africa around 300,000 years ago, as the earliest Homo sapiens were evolving, a new study finds.

This discovery indicates that a separate Homo population, perhaps a species some researchers call H. heidelbergensis (SN: 6/22/19), inhabited Africa at the same time as both H. sapiens and a recently discovered population dubbed H. naledi (SN: 6/10/17), say geochronologist Rainer Grün and his colleagues. African H. heidelbergensis could have been a recently reported “ghost population” (SN: 3/14/20) that interbred with ancient H. sapiens and passed a small amount of DNA to present-day West Africans, the researchers suggest April 1 in Nature.

“We can now identify at least three distinct and contemporary [Homo] lineages in Africa about 300,000 years ago, but we don’t yet know whether our ancestry was largely or entirely contained within the H. sapiens part of that variation,” says paleoanthropologist and study coauthor Chris Stringer of the Natural History Museum in London.

Researchers have puzzled over the age of the Broken Hill skull since its 1921 discovery in south-central Africa. Metal ore mining at what was then Northern Rhodesia’s Broken Hill mine revealed deposits bearing the skull and two associated leg fossils. The site, located in what’s now known as Zambia, has been named Kabwe. Previous age estimates for the fossils, based on clues such rodent fossils and stone tools found at the site, have ranged widely from around 500,000 to 125,000 years old.

Because quarrying destroyed the site, sediment that may have yielded fossils can’t be dated. Instead, Grün, of Griffith University in Nathan, Australia, and his team dated small samples of bone and teeth from the Broken Hill skull using measures of the radioactive decay of uranium and the accumulation of natural radioactivity from sediment and cosmic rays. Based on these techniques, the team estimates the skull’s age at between 324,000 and 276,000 years old.

Double-edged stone implements typically found at ancient H. sapiens sites were also recovered near the Broken Hill skull, suggesting that H. heidelbergensis made the same type of tools, the scientists say.

It’s unclear if H. heidelbergensis fashioned those artifacts, which also can’t be definitively linked to the Broken Hill skull, counters archaeologist Eleanor Scerri of the Max Planck Institute for the Science of Human History in Jena, Germany.

But a “seemingly reliable age of approximately 300,000 years” for H. heidelbergensis in Africa fits a scenario in which H. sapiens began evolving across Africa (SN: 12/21/19) around that time , Scerri says. In that scenario, mating occurred among dispersed human populations with various skeletal traits, as did occasional interbreeding with other Homo species. “It wouldn’t be surprising if there was some gene flow between the lineage leading to us and H. heidelbergensis,” she says.

African Homo fossils from around 300,000 years ago (SN: 7/8/17), including the Broken Hill skull, are either direct ancestors or close relatives of H. sapiens, says paleoanthropologist María Martinón-Torres of the National Research Center on Human Evolution in Burgos, Spain. But H. heidelbergensis has become a confusing species designation, comprising a diverse group of partial hominid fossils that can’t easily be compared to one another, she contends.

Populations in many parts of Africa may have mixed and mingled to produce H. sapiens, “but at this point the picture is still blurred,” Martinón-Torres says.