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An ancient human that lived in China at least 160,000 years ago had an unusually large brain for the time 鈥� comparable to the brain size of people alive today. The find is more evidence that hominin evolution went in many different directions, rather than taking a straight line from small brains to large ones.

It is also possible that the skull belonged to a mysterious kind of hominin called a Denisovan. Very few Denisovan bones are known, so a new one would be a significant find. There is evidence that the Denisovans were particularly big-bodied, in which case proportionally large brains would be expected.

The newly described bones come from Xujiayao, an archaeological site in northern China that was first excavated in the 1970s. Down the years, the site has yielded 21 fragments of hominin skeletons, ranging from teeth and bits of jawbone to pieces of skull. 鈥淭hese fragments represent at least 10 individuals,鈥� says of the Chinese Academy of Sciences in Beijing.

Wu and his colleagues have now found that three of the skull fragments fit together neatly, suggesting they all belonged to the same individual. They call this hominin Xujiayao 6, or XJY 6. The sediment in which it was found is .

The researchers have calculated the volume of the hominin鈥檚 brain from the size and shape of the skull fragments. Their estimate is that it was between 1555 and 1781 cubic centimetres, with a figure nearer to 1700 cubic centimetres most likely.

That is unusually large for the time, says Wu. Back then, hominin brains had an average volume of 1200 cubic centimetres. Instead, Xujiayao 6 is on a par with later modern humans and Neanderthals, some of whose brains were 1700 cubic centimetres聽or a little larger.

The analysis looks like it was carefully done, says at Florida State University in Tallahassee. 鈥淲e can probably believe it鈥檚 a reasonable estimate of the cranial capacity.鈥�

Over the past 7 million years, there has been an overall trend of increasing brain size among hominins 鈥� but that trend hides a lot of complexity and there are exceptions to the rule.

For Wu, Xujiayao 6 is 鈥渢he earliest evidence of a brain size that falls in the upper range of Neanderthals and modern Homo sapiens鈥�. But while Xujiayao 6 belonged to a big-brained population, other hominins around at the same time had very small brains. The 鈥渉obbits鈥� (Homo floresiensis) who lived on Flores in Indonesia until about 50,000 years ago had brains measuring just 400 cm³, while in South Africa 350,000 years ago, Homo naledi had brains measuring 560 cm³. Even early members of our own species (Homo sapiens) did not have brains as large as living people: H. sapiens skulls from a 315,000-year-old site in Morocco .

However, Xujiayao 6鈥檚 big brain tells us little about its mind, says at the University of the Witwatersrand in Johannesburg, South Africa. 鈥淲e are long past the days when brain size was the famous 鈥榗erebral Rubicon鈥� used to assign human status,鈥� she says. Big brains aren鈥檛 necessarily linked to intelligence. 鈥淗uman populations in colder environments have larger brains,鈥� she says, but that is because of 鈥渢hermal packaging鈥� that keeps the neurons warmer and thus able to function.

Falk agrees that intelligence isn鈥檛 closely linked to brain size. 鈥淚t鈥檚 really largely about the wiring and the neurochemistry and the internal organisation of the brain and distributed networks and all that,鈥� she says.

What kind of hominin was Xujiayao 6? For Wu, it is probably something new. 鈥淪ome new type of Homo, or new species, existed in the late Pleistocene,鈥� he says.

With only three fragments of skull to go on, a firm identification is impossible, Falk says. 鈥淚鈥檓 wary to point to a specimen and try to identify the species.鈥�

at the University of Toronto in Canada says there is a simpler alternative to naming a new species.

A hominin group called the Denisovans existed in east Asia at this time. They were first identified using DNA from fragmentary remains in Denisova cave in Russia, and have since been found on the Tibetan Plateau. The few known bones look unusually large, and the teeth from Xujiayao have been speculated to be Denisovan because they are also big.

鈥淗onestly, this is probably the same stuff,鈥� says Viola. 鈥淯ntil we have DNA, we鈥檒l never 100 per cent know, but it鈥檚 obviously a good fit.鈥�

If Xujiayao 6 is a Denisovan, its large brain could be explained: if they were indeed big-bodied, their brains would be proportionately large as well.

It isn鈥檛 the first time a potential Denisovan has shown up. Last year, a partial skull from China known as the Harbin cranium was described. Some of the researchers involved argued it belonged to a new species, which they called Homo longi. But others think it was probably a Denisovan. 鈥淭hat鈥檚 the obvious conclusion,鈥� says Viola. He adds that Xujiayao 6 and the Harbin cranium 鈥渓ook relatively similar鈥�.

Wu is open to the idea. 鈥淚t is a possibility that the XJY 6 is related to the Denisovans,鈥� he says. But because we don鈥檛 yet have confirmation of what Denisovan skulls look like, 鈥� this needs further evidence to prove it鈥�.

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