In September 2018, China Daily ran a story that found its way to my desk. It featured the photograph of a beautifully preserved fossil skull with huge brow-ridges, with a caption speculating – wrongly, as it would turn out – that it belonged to a species of human known as Homo heidelbergensis.

The article told a remarkable story about the discovery of this fossil some 85 years earlier. Japanese forces had been occupying the city of Harbin in north-eastern China in 1933. They ordered a team of local workers to help build a railway bridge across the Songhua River.

The workers uncovered the skull in river deposits, and one of them hid it from the Japanese by lowering it down a disused well. Feeling ashamed of having worked for the enemy, he kept quiet about the find for many years. Finally some 80 years later he told his son and grandson as he lay dying, which led to the almost intact skull being recovered and later donated to Hebei Geo University near Beijing.

That origin story is difficult to verify, and there were rumours that the fossil was actually a fake, but I was able to confirm first hand that it was real when I was invited to join Chinese researchers in studying this astonishing relic at the end of 2018. I jumped at this wonderful opportunity to help put one of the most complete human fossils ever found in its proper place in human evolution.

So many ways to be human

Establishing that place has got a lot trickier in the last 20 years or so. We think that humans (or hominins) split from the ancestors of our closest living relatives, the chimpanzees, about 7 million years ago. For the next 5 million years or so, the hominins continued their evolution in Africa, and about 2 million years ago, began to disperse across Asia and Europe.

It used to be thought that human evolution then progressed from more primitive forms through to modern humans (Homo sapiens), either in just one region, most likely east Africa, or across the whole inhabited world, following the early exit from Africa. Now, we know it wasn’t so simple.

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There was a profusion of different kinds of humans, sometimes living at the same time and even in the same regions, as nature conducted multiple experiments over millions of years in how to become human. We are the sole survivors of all those experiments.

Even during the last 300,000 years the number of human species has been startling. The ancient Homo heidelbergensis, with their heavy-set physiques and large faces and jaws, was surviving in Europe – along with similar people in Africa – 300,000 years ago. The even more ancient Homo erectus, which had a smaller brain and angular flat-topped skull, was clinging on in parts of Indonesia until at least 100,000 years ago.

Homo neanderthalensis (Neanderthal human), with a broad thick-set body and protruding midface around a large nose, was living in western Eurasia from at least 400,000 to 40,000 years ago. We Homo sapiens – first given that name in 1758, meaning “wise human” – existed at least 300,000 years ago, first in Africa and then globally within the last 50,000 years.

In addition, there are three enigmatic small-bodied forms (and also known to be small-brained in the first two cases) from the same recent period: Homo naledi (star human), Homo floresiensis (Flores human) and Homo luzonensis (Luzon human). These lived respectively in South Africa about 300,000 years ago; on the island of Flores, eastern Indonesia, from at least 190,000 to 50,000 years ago; and on the island of Luzon, the Philippines, about 130,000 years ago. All three seem to be long-lasting relics of even more ancient stages of........

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