The elegant pear-shaped, double edged tool, known as the Acheulean ‘hand-axe’ is an icon for the distant past of humans. It appears in the record as a sharp contrast to the earlier crude cutting tools made of broken and sharpened pebbles, known generally as Oldowan, that around 2.5 Ma marked the appearance of some hominin species with the wit to exploit the inorganic world and begin manufacture. There can be little doubt that the visualisation of a useful shape within a formless block of stone and the dexterity to realise it as a tool marked a major change in human cognitive ability. Oddly, there is no agreement on what the bi-face axe was for. Even odder is the frequent discovery of pristine examples that seem never to have been used. It has long been accepted that the originator was Homo erectus living in Africa around 1.5 to 1.6 Ma ago. It also seems likely that the first migrants out of Africa were H. erectus at around 1.7 to 1.8 Ma, who reached Europe and Asia, surviving in SE Asia until perhaps as recently as 20 ka. Yet the earliest émigrés seem not to have carried the bi-face technology with them (but see: Early bi-face tools from South India in EPN May 2011); an absence from the record that has been taken to indicate that they left before its invention. That assumption is no longer as sound as it was formerly.
The shores of Lake Turkana in NW Kenya have provided rich pickings for ancient tools, both Oldowan and more rarely Acheulean, and at Nariokotome was found an almost complete skeleton of a young H. erectus: ‘Turkana Boy’. Some of the most prolific sites are around Nachukui in a thick sequence of terrestrial and lacustrine sediments that span the period 2.3 to 1.6 Ma. Volcanoclastic sediments that are datable by radiometric means are few and far between, but the base of the sequence, 100 m below the stratigraphically oldest bi-face tool-bearing horizon, is a tuff with an Ar-Ar age of 2.33 Ma, with another 20 m below aged 1.87 Ma. A team of French and US archaeologists (Lepre, C.J et al. 2011. An earlier origin for the Acheulian. Nature, v. 477, p. 82-85) used systematic measurements of magnetic polarity through the intervening sediments to match this magneto-stratigraphy with the global record of flips in the geomagnetic field, famed for its use in unravelling plate tectonics through magnetic ‘stripes’ above the ocean floor. The match is extremely good, and the Acheulean occurrence is a mere 5 m above the end of the Olduvai subchron. Plotting radiometric and magneto-stratigraphic datums against sediment depth gives a calibration that suggests an age for the tools of 1.76 Ma.
Fortuitously, this age coincides with the earliest evidence for out-of-Africa human migration. So why did the early travellers not equip themselves with this literally ‘cutting-edge’ technology? The answer may lie in the close association of sites bearing bi-face tools and those devoid of them. The technology was not universally used; indeed, some may have been ‘imported’ from the originators (but not shared). Another possibility is that the archaeological record marks two species, one with and the other without bi-face tools. Also, Lake Turkana is far distant from points at which people could have made the crossing to Europe and Asia; the diffusion of the new may not have reached that far. Easily as important is the near coincidence of the date with that of the earliest known African H. erectus remains. Their speciation, involving an increased cranial capacity seems to have coincided with new ways of using that potential. Whatever use the bi-face tools had, they and the brain that made them conferred such an advantage that there was little evolution in both humans and their technology for more than a million years until H. heidelburgensis appeared; revolutionary change followed by prolonged biological, social and intellectual conservatism.
- Humans shaped stone axes 1.8 million years ago, study says (eurekalert.org)