Australasian Science: Australia's authority on science since 1938

Climate Biases Fossil Record of Early Humans

The fossil record of early humans from the Cradle of Humankind caves in South Africa is biased towards periods of drier climate, according to research published in Nature (https://goo.gl/GTEjTv). This finding suggests there might be gaps in the fossil record, potentially obscuring evolutionary patterns and affecting our understanding of both the habitats and dietary behaviours of early hominins in this region.

The Cradle of Humankind is a World Heritage site north-west of Johannesburg. It’s the world’s richest early hominin site, and home to nearly 40% of all known fossils of our human ancestors. However, the caves have collapsed over the years, making dating of the fossils difficult.

“While the South African record was the first to show Africa as the origin point for humans, the complexity of the caves and difficulty dating them has meant that the South African record has remained hard to interpret,” explained co-author Prof Andy Herries of La Trobe University’s Department of Archaeology and History, who conducted excavations at many of the sites dated in the study.

Instead the researchers measured trace radioactive isotopes in 28 thick “flowstones” – stalactites and stalagmites that form in caverns when flowing waters deposit dissolved minerals. Prof Jon Woodhead of the University of Melbourne said the research findings show that the fossils in “the Cradle caves date to just six specific time periods between about 3.2 and 1.3 million years ago”.

The caves were likely to be inaccessible when the land was wetter, so the ages of the flowstones date gaps in the fossil record when the caves were more likely to be closed off to incoming sediments and hominin remains.

“We know that significant flowstones only grow in caves during wet periods, when there is more rain outside the caves,” explained lead researcher Dr Robyn Pickering of the University of Cape Town. “By dating the flowstones, we are picking out these times of increased rainfall. We therefore know that during the times in between, when the caves were open, the climate was drier and more like what we currently experience.”

This means the early hominins living in the Cradle experienced significant changes in local climate, from wetter to drier conditions, at least six times between 3.2 and 1.3 million years ago. When the climate became drier, however, vegetation cover would have diminished, increasing surface erosion and opening the caves to outside sediments and the preservation of hominin remains.