500,000-year-old shell cutting tool could explain Acheulean hand-axe puzzle
The teardrop shaped Acheulean hand-axe is without doubt the signature artefact of the Lower Palaeolithic, remaining in use for nearly one and a half million years. The tools first came into use about 1.75 million years ago and persisted until around 250,000 years ago. While chiefly associated with Homo erectus, they remained in use into the time of the larger-brained Homo heidelbergensis.
What has long been a puzzle is that while the hand-axes are ubiquitous in Africa, Europe and Southwest Asia, they are very rare further east. The boundary between the two regions is known as the Movius Line, after the American archaeologist Hallam Movius who first noted the discontinuity in 1948. The Movius Line has largely stood the test of time: the occasional Acheulean-like artefacts that have been found in China and South Korea are probably due to an eastwards migration of later hominins about 250,000 years ago (Cameron & Groves, 2004). By this time, hand-axes had been abandoned in the West (Klein, 2005).
The simplest explanation is that the ancestors of Homo erectus living east of the Movius Line had already left Africa by the time the hand-axes were invented (Swisher, et al., 1994). Another possibility is that not every hominin group in Africa adopted Acheulean technology, and that the original migrants were among those lacking it (Lepre, et al., 2011). Both possibilities suppose that firstly, the hand-axes were only invented once (or were not re-invented by groups that had left Africa); and that secondly, after the initial colonisation of Southeast Asia and China, the migrants there had no significant contact with the Acheulean-using peoples living elsewhere. Another possibilities is that the population sizes at the extremities of the migration were too low to keep the skills necessary for hand-axe production alive from one generation to the next (Lycett & von Cramon-Taubadel, 2008). None of these explanations are very compelling, because it is now known that prepared-core techniques were independently invented in Africa and Eurasia (Adler, et al., 2014), and it therefore seems unlikely that the simpler Acheulean technology arose only once.
A more likely explanation is that migrants passed through a region lacking suitable raw materials to make the hand-axes and were forced to look to alternatives for tool-making. One suggestion is they switched to bamboo and either forgot how to make hand-axes or were happy to stick with bamboo even when suitable stone was available (Klein, 2005). It is also possible that they switched to bamboo because it is readily available and an excellent tool-making material in its own right (Cameron & Groves, 2004; Roberts, 2009; Lewin & Foley, 2004).
It has long been claimed that the theory cannot be proved because bamboo implements from such a long time ago are extremely unlikely to have survived – but there are other alternatives to stone that can survive for hundreds of thousands of years. The recently-discovered shell cutting tool from Trinil, Java, is around 500,000 years old, and it was found in a region where suitable stone is scarce (Joordens, et al., 2014). Although it is far more recent than the first eastwards migration of Homo erectus, it demonstrates that the species was capable of utilising alternative raw materials for tool-making where necessary. It therefore seems likely that the Movius Line represents a shift to the use of organic materials for tool-making when stone suitable for hand-axe manufacture was unavailable.
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