Dating the Broken Hill skull: Homo heidelbergensis was younger than we thought

When the Broken Hill skull was first discovered in what is now Kabwe, Zambia, it was quickly realised to have belonged not to a modern human, but an ancient one.

While initially it was named Homo rhodesiensis, it has since been classified as one of the best preserved fossils of another ancient human species called Homo heidelbergensis. Thought to have first appeared some 600,000 years ago, H. heidelbergensis spread throughout much of Africa and Europe, leaving in their wake a smattering of fossils and plenty of stone tools.  

Pinning an exact date on the Broken Hill fossil has been difficult, because the site from which it was found has since been destroyed. But it was long thought to have been roughly 500,000 years old, tallying up nicely with the other dates known for the species.

Prof Chris Stringer, a researcher in human evolution at the Museum, has worked with geochronologist Rainer Grün and other colleagues to produce the best estimate for when the owner of the Broken Hill skull died.

Chris says, 'Through years of painstaking work including direct dating of the skull itself and other human and non-human materials found around the Broken Hill site, we have produced a best age estimate of about 299,000 years for the Broken Hill skull.

'This is surprisingly young.'

The new age for the fossil changes what we thought was happening on the African continent, not only when H. heidelbergensis was living and dispersing across the landmass but also when our own species, Homo sapiens, was evolving prior to making its forays into the rest of the world.

The study has been published in the journal Nature. 

A ground-breaking discovery

The skull was discovered in a lead and zinc mine at Broken Hill, in what was then known as Northern Rhodesia but is now Kabwe in Zambia.

It was unearthed by an unnamed African miner and his Swiss colleague called Tom Zwigelaar . Elsewhere in the site, various other fossils were found, including fragments of femur and the partial jaw of another individual. These are not the only fossils to have come out of the Broken Hill mine, as records show that it was rich in the remains of animal bones.

The human fossils were noted for their importance as the first significant hominin fossils found in Africa. They were donated by the Rhodesia Broken Hill Mine Company who owned the mine to what was then the British Museum but is now the Natural History Museum, London in 1921.

When the fossils arrived in London they were examined by the palaeontologist Arthur Smith Woodward, who declared that they belonged to a new species of human, which he named Homo rhodesiensis.

More recently, many researchers have instead suggested that these fossils represents variation within another ancient human species known as Homo heidelbergensis. 

The fossil remains of H. heidelbergensis, in the form of bones and stone tools, have been found in many parts of Africa, as well as much of Europe. Several of those other fossils dated to around 500,000 years ago, giving rise to an intriguing theory.

'Until recently I - and many other palaeoanthropologists - argued that H. heidelbergensis existed about 500,000 years ago, and was probably the last common ancestor of our species H. sapiens evolving in Africa, and Homo neanderthalensis (the Neanderthals) which evolved in Eurasia,' says Chris.

This new piece of research carried out by Chris and his colleagues suggests that the picture was much more complex than this.

A cosmopolitan planet

Dating the Broken Hill skull has proved difficult, largely because the mine in which it was discovered has long been quarried away.

Originally, comparisons of the fossil with other sites in Africa and Europe led researchers to presume the skull was likely around 500,000 years old, but this has never been confirmed.

Chris and his colleagues turned to uranium isotopes found within the Broken Hill skull itself to pinpoint a far more accurate date. This yielded an age of about 299,000 years ago, within a range of 25,000 years each side.

This is surprising because until recently it was thought that by this point in time there was probably only one species of hominin living on the African continent – Homo sapiens. 

'We already knew that Eurasia contained diverse human lineages about 300,000 years ago,' explains Chris. 'Now, the same applies to Africa, since H. heidelbergensis must have been contemporary with more sapiens-like fossils in Morocco and Kenya and the much more primitive Homo naledi, recently discovered in South Africa.

'A date of about 300,000 years old highlights the complexity of human evolution in Africa.'

It is now looking like Africa and Eurasia were inhabited by a whole range of hominin species just a few hundred thousand years ago. While H. naledi was living in South Africa, H. heidelbergensis was surviving in South-Central Africa, and H. sapiens was emerging in Morocco and Ethiopia.

At the same time as all this H. neanderthalensis was evolving in Europe, the Denisovans were developing in Asia, Homo erectus may still have been clinging on in Indonesia, and two diminutive hominins, Homo floresiensis and Homo luzonensis, were living the island life in Southeast Asia.

It seems that the world was a busy place, and we're only now starting to understand what this may mean for our own origins.