A little over two decades ago, when the new millennium began, it seemed that the traces left by our ancient human ancestors, dating back more than about 50,000 years, were exceedingly rare.
Only four sites were reported in all of Africa at that time. Two were from East Africa: Laetoli in Tanzania and Koobi Fora in Kenya; two others were from South Africa (Nahoon and Langebaan). In fact, the Nahoon site, reported in 1966, was the first site ever described.
In 2023 the situation is very different. It seems people weren’t looking hard enough or weren’t looking in the right places. Today, the African tally for hominid ichnosites (a term that includes both footprints and other traces) dated older than 50,000 years is 14. These can be conveniently divided into an East African cluster (five sites) and a South African group (nine sites). There are a further ten sites elsewhere in the world, including the UK and the Arabian Peninsula.
Given that relatively few hominid remains have been found on the South African coast, the traces left by our human ancestors as they moved across ancient landscapes are a useful way to supplement and improve our understanding of ancient hominids from Africa.
In a recent article in Ichnos, the international journal of trace fossils, the ages of seven hominin ichnosites newly dated within the past five years on the southern coast of South Africa have been provided. These sites are now part of the ‘South African cluster’ of nine sites.
Sites were found to vary by age; the most recent is about 71,000 years old. The oldest one, which dates back to 153,000 years, is one of the most remarkable discoveries, moreover it is the oldest fingerprint so far attributed to our species, Homo sapiens.
Very different sites
There are significant differences between the East African and Southern African land groups. The East African sites are much older: Laetoli, the oldest, is 3.66 million years old, and the youngest is 0.7 million years old. The tracks were not made by Homo sapiens, but by earlier species such as australopithecines, Homo heidelbergensis and Homo erectus. For the most part, the surfaces on which the East African traces are located had to be laboriously and meticulously excavated and exposed.
The South African sites, on the other hand, are substantially younger. All have been attributed to Homo sapiens. And the tracks tend to be fully exposed when they are discovered, in rocks known as eolianites, which are the cemented versions of ancient dunes.
Excavation is therefore not usually considered – and due to the site’s exposure to the elements and the relatively coarse nature of the dune sand, they are usually not as well preserved as East African sites. They are also vulnerable to erosion, so you often have to work quickly to record and analyze them before they are destroyed by ocean and wind.
Although this limits the potential for detailed interpretation, we can date the deposits. That’s where optically stimulated luminescence comes in.
A method of illumination
A key challenge when studying paleotracks, fossils, or any other type of ancient sediment – is determining the age of the materials.
Without it, it is difficult to assess the wider significance of a discovery or to interpret the climate changes that create the geological record. For eolianites from the South African coast, the dating method of choice is often optically stimulated luminescence.
This dating method shows how long ago a grain of sand was exposed to sunlight; in other words, how long that section of sediment was buried. Given the way the tracks in this study were formed—prints made in wet sand, followed by burial with freshly blown sand—it’s a good method, as we can be reasonably confident that the encounter “clock” began ca. at the same time it was created.
The overall date range of finds for hominid ichnosites—about 153,000 to 71,000 years old—is consistent with ages in previously reported studies from similar geological deposits in the region.
The 153,000-year-old track was found in the Garden Route National Park, west of the coastal town of Knysna on Africa’s southern coast. The two previously dated South African sites, Nahoon and Langebaan, were approximately 124,000 years old and 117,000 years old, respectively.