Chose the top bunk (Image: Frans Lanting/Corbis) Chimp nest on the ground (Image: K. Koops) Human apes chimp in ground nest (Image: K. Koops)


WHERE did early humans bed down for the night? Evidence that some chimps routinely eschew the safety of treetops to sleep on the ground raises the possibility that some early hominins did too – with possible implications for their cognitive development.

By about 3.2 million years ago Australopithecus afarensis had arches in the soles of its feet – a key adaptation for walking on the ground. But its long arms and slender fingers suggest to some researchers that it was still comfortable with climbing.

That skeleton makes sense if australopithecines slept in trees at night to escape predators, as chimps do today. If that was the case, hominins may not have slept on the ground until Homo erectus appeared 1.9 million years ago. They lacked upper body adaptations for climbing and may have used fires to ward off ground-dwelling nocturnal predators, although the evidence for controlled fire stretches back only 1 million years.

Now, evidence from west African chimps (Pan troglodytes verus) of the Nimba mountains in Guinea suggests ground-sleeping may predate such innovations. Of the 634 sleeping nests that Kathelijne Koops of the University of Cambridge found in Nimba, 90 were built on the ground. With colleagues at Cambridge and Kyoto University in Japan, Koops collected hairs from 46 ground nests. DNA analysis showed that at least 12 chimps had slept on the ground, suggesting it is a widespread behaviour (American Journal of Physical Anthropology, DOI: 10.1002/ajpa.22056).

The chimps of Nimba have few predators, but similar ground nests are seen in the Democratic Republic of the Congo, where leopards are also present. “Ground nesting can become established despite the presence of predators and without the use of fire,” says Koops, adding that this suggests australopithecines may have slept on the ground too.

Carol Ward at the University of Missouri in Columbia points out that there are too many differences between chimps and early hominins to draw firm conclusions about early human behaviour from chimp studies.

Tim White at the University of California, Berkeley, is comfortable with the idea of ground-sleeping hominins, though – largely because other evidence points that way. Upper-body evidence notwithstanding, he says, the anatomy of A. afarensis shows that its ability to climb trees was compromised. “It’s nice to have a study that indicates that [ground-sleeping] is not necessarily related to the adoption of fire,” he says. Gorillas also routinely sleep on the ground, he adds.

The period that hominins began sleeping on the ground may have been pivotal for their cognitive development, says Thomas Wynn at the University of Colorado in Colorado Springs. It allowed them to spend more of the night in REM sleep, which he says is important for memory consolidation and cognition. A common feature of REM sleep is muscle paralysis, which makes it precarious for apes that sleep in trees, says Wynn.

Koops agrees a feedback loop may have been at work: ground-sleeping could have provided cognitive boosts that helped hominins to ward off nocturnal predators – like mastering fire – allowing for even more restful sleep and further cognitive development. Proving any of this will be difficult, says Ward. “This topic is far into the realm of speculation.”