We know from discoveries at Niah Cave that early modern humans were exploiting rainforests on the island of Borneo in South East Asia about 40,000 years ago. However, new evidence published in Nature in August 2017 suggests that they had already arrived in the region at least 63,000 years ago, far earlier than expected.

The finds in question were actually made between 1887 and 1890 by Eugène Dubois, the famous Dutch discoverer of the first Homo erectus fossils, when he was investigating sites in Sumatra, before switching his focus to Java. However, it was not until 1948 that Dutch palaeontologist Dick Hooijer identified a human incisor and molar tooth in Dubois's collection from Lida Ajer cave. But the exact location of the cave was unknown until it was rediscovered, using Dubois's notes, by a team lead by Kira Westaway from Macquarie University, Sydney. A restudy of the Dubois materials including the human teeth began, along with a programme aimed at dating the levels from which the teeth and associated finds had come.

Eugène Dubois (1858 - 1940) is often considered one of the founding fathers of modern palaeoanthropology. It was Dubois's aim to find the supposed "missing link" between apes and humans, and he decided to look for this transitional form on the Sunda islands, which were part of the Dutch East Indies at that time. One of the first places he looked was Sumatra where Dubois explored a number of caves in the Padang Highlands, including the Lida Ajer cave. He found numerous fossils but realized that they were too young to contain an ancestral human. He soon lost interest in Sumatra and switched his attention to nearby Java where he finally achieved fame by discovering the remains of an archaic human, "Pithecanthropus erectus" or "Java Man", now classified as Homo erectus (Image credit: public domain).

Using the latest research techniques, including micro-CT, our team were able to conclusively show that the teeth were morphologically modern human, through detailed study of the crown morphology and internal traits such as enamel thickness and the junction between the enamel and dentine. Furthermore, the application of Uranium-series, Luminescence and Electron Spin Resonance dating methods to the associated sediments and finds was able to place the teeth at an age between 63,000 and 73,000 years old.

This adds to a growing picture that Homo sapiens was spreading through Asia towards Australia before the date of ~60,000 years usually assigned to the main Out of Africa dispersal. We know that early modern humans were in Israel >100,000 years ago, and there are controversial claims of modern-looking teeth from Fuyan Cave, China dated to at least 80,000 years, and a modern human tooth at Punung in Java dated at more than 100,000 years old. A pre-60,000 year old presence of modern humans in South East Asia is also inferred by the recent dating of artefacts and pigments at Madjedbebe rockshelter in northern Australia to about 65,000 years old.

Entrance to the Lida Ajer cave in the Padang Highlands of central Sumatra. Lida Ajer is a limestone cave harbouring fossils of Pleistocene origin that are associated with a rich rainforest fauna. Two human teeth originally discovered here by Dubois have now been dated to 63,000 to 73,000 years old (Image credit: Chris Stringer, The Natural History Museum, London).

However, if H. sapiens was already widespread in South East Asia more than 60,000 years ago, this raises yet other questions. These include the apparent survival of archaic humans in Eurasia and S.E. Asia (Neanderthals, Denisovans, H. floresiensis, and perhaps also H. erectus) after this date, when many scientists would have expected them to have been supplanted. Perhaps the numbers of modern humans were small early on, they moved faster and in narrow zones (maybe coastal?), or their ability to replace the other populations was more limited at this earlier date. Countering those suggestions is the fact that the Lida Ajer humans were clearly present well inland, in challenging rainforest environments, while the Madjedbebe early Australians were seemingly long-term occupants of the site, and were technologically sophisticated.

These latest findings suggest that either the genetic calibrations estimating the main Out-of-Africa dispersal at about 60,000 years ago are wrong, or there were indeed earlier migrations of modern humans spreading far from our African homeland, but these were supplanted by later dispersals. Perhaps the earlier colonisers went largely or completely extinct, or they were absorbed by subsequent larger population movements, which overwrote the genetic signals of their presence.