Culture in the most general sense that encompasses tools, clothing, habitation and fire has increasingly set humans and their ancestors apart from the rest of the natural world. It might therefore seem that becoming more ‘human’ cushions our line from Darwinian natural selection since we have created our own ‘nature’ and carry it with us. Setting fully modern humans adrift in the environment, without that culture, would undoubtedly result in rapidly extinguishing the species. In that hypothetical context we are far from ‘fit’, in Darwin’s sense. However, the development of humanity’s cultural milieu has itself provided a continually changing, increasingly pervasive artificial set of conditions for natural selection. Culturally, the most dramatic step in human evolution, for which we have tangible evidence, emerged with the explosive appearance of graphic art and a complex ‘toolkit’ around 35 thousand years ago in Europe. That huge advance will undoubtedly be traced back maybe tens of millennia when archaeological finds in Africa and Australia, for instance, are more precisely dated. Evidence from the DNA in male-carried Y chromosomes indicates that a profound genetic shift occurred around 70 ka, perhaps resulting from a decline in global human numbers to a very small population after the climatic disaster wrought by the explosive eruption of the Toba volcano in Indonesia. That too was a time when fully modern humanity distributed itself more thinly by a decisive exodus from Africa. Some specialists have speculated that the cultural explosion stemmed from that evolutionary ‘bottleneck’. There are genetic signs of adaptation to cultural practices and selective pressures that accompanied them after the rise of agriculture and settlement (See Has human evolution stopped?, September 2005 issue of EPN). Recent work on the whole human genome gives an inkling that even more pervasive evolutionary changes took place in the last 50 thousand years (Wang, E.T. et al., 2005. Global landscape of recent inferred Darwinian selection for Homo sapiens. Proceedings of the National Academy of Science, www.pnas.org/cgi/doi/10.1073/pnas.0509691102).
Wang and colleagues from the University of California studied the occurrence of single-letter differences in the genetic code (single-nucleotide polymorphisms – SNPs). Scattered across all human chromosomes are about 1.6 million of these SNPs. They appear not to do anything, but can be linked to nearby genes. When natural selection favours a particular mutated variant of a gene, the associated SNPs can be selected as well. The approach used by Wang et al. is a statistical search for pairs of SNPs that occur together more often than could be possible by chance ‘reshuffling’ that occurs from generation to generation. Their analysis suggests that around 1800 genes, a remarkable 7% of the whole genome, have changed over the last 50 thousand years. Interestingly, that is similar to the degree of genetic change in maize since its domestication from its wild ancestor. As well as genes connected to protein metabolism that could have changed as new diets followed the rise of agriculture, some that are involved in brain function have been selected as well.
Although at an early stage, this kind of research confirms that we are indeed still evolving along Darwinian lines, perhaps unwittingly domesticating ourselves. It is easy to assume that ideas, skills and artistic sensibilities are passed on through language and learning and thereby grow and diversify, but in order for any of these to stimulate the deep feelings that they foster suggests that some aspects have become ‘hard-wired’ in all of us. Everyone unconsciously taps their feet to rhythm, can be moved to a vast range of emotions by music, words and visual stimuli, and can ‘sense’ an environment captured, even in abstraction, by a talented artist. They inspire further development. Until around 50 ka human culture, insofar as we can see evidence for it, remained fixed for more than a million years through several species and subspecies of the genus Homo. Appearing between 1.6 and 1.4 Ma ago the bi-face stone axe endured as humanity’s highest known achievement until those very recent times.
See also: Holmes, R. 2005. Civilisation left its mark on our genes. New Scientist, 24/31 December 2005 issue, p. 8.
Earliest tourism in northern Europe
Some years ago British palaeoanthropologists were in a state of high excitement about finds of stone tools, evidence of prolonged human habitation and fragmentary skeletal remains from a sandpit at Boxgrove on England’s southern coast. They showed the earliest human presence at high latitudes around 400-500 ka. The date of early colonisation has now been pushed back more than half as long before that to 700 ka by finds in a shoreline exposure of riverine sediments on the coast of Suffolk on England’s east coat. The Cromer Forest Bed of Middle Pleistocene age has been know since Victorian times as a rich source of the flora and fauna from one of the earliest interglacials of the current period of 100 ka climate cyclicity. At that time the North Sea had yet to establish a connection that would eventually separate the British Isles from Europe, and the site at Pakefield would have been the estuary of a now-vanished river system draining the Midlands and Wales. So far no human bones have turned up in the excavations, which have to be conducted at low tide. But many flint tools pepper the organic-rich sediments (Parrfitt, S.A. et al., 2005. The earliest record of human activity in northern Europe. Nature, v. 438, p. 1008-1012). As with most terrestrial deposits, establishing the age of human occupation posed the greatest difficulty. A careful documentation of magnetic polarity combined with fossils – including distinct voles – and a new technique that relies on assessing the degree of protein degradation in bivalve shells helped tie-down the age precisely.
Around 800 ka human occupation had begun in Spain and the Pakefield site shows that migration northwards of flora and fauna following a glacial epoch was swift, to establish conditions considerable warmer than in the Holocene. It seems that this Mediterranean climate encouraged such northward penetration by humans, most likely during a short period of particular warmth. Long eyed by archaeologists as a potential source of human remains, patience has paid off in the Cromer Forest Beds. Yet around the world there are many other, equally promising strata or Pleistocene age that have not had such undivided attention for so long, A glance at the distribution of keynote sites for palaeoanthropology shows how narrow the search for human origins and migratory destination has been up to now. Though it is understandable that once finds have been made, funds and scientists cluster where progress is best guaranteed. Very rarely, either a ‘shot in the dark’ pays off or something surprising turns up at a site being excavated for other purposes. Broadening the search may well have high financial and career risks, yet the more discoveries are made at well-trodden sites the greater the likelihood that the full story of human evolution and migration will be revealed by breaking new ground,
See also: Roebroeks, E. 2005. Life on the Costa del Cromer. Nature, v. 438, p.921-922.
Biogeochemical evidence for vegetation change when hominins evolved
A long-held theory that concerns the background to hominin evolution, is that the freeing of hands by bipedalism was triggered by a shift in the ecology of East Africa from forest to more open grassland. That might well have happened as the Neogene uplift associated with development of the East African Rift transformed the regional wind and rainfall patterns to the way they are today, thereby creating the conditions for the modern savannahs and semi-deserts in the area long associated with human origins. The lakes of East Africa are ephemeral in the context of Neogene climate change, and so their sediments are not much use in charting long-term shifts in flora. However, the modern wind systems shift dust and organic particles consistently towards the Gulf of Aden, so sediment cores there potentially provide a continuous record of vegetation change. That is, if they contain ‘biomarkers’ that distinguish the debris of trees from that of grasses. The first biomarker records from the Gulf of Aden seabed powerfully confirm the notion of vegetation change as a possible driver for hominin evolution (Feakins, S.J. et al., 2005. Biomarker records of late Neogene changes in northeast African vegetation. Geology, v. 33, p. 977-980).
Up to about 3.5 Ma the cores contain plant-derived waxes that are characteristic of trees that use C3 metabolic processes, but thereafter evidence for increasing C4 grasses predominates. Coinciding with that broad trend is an increase in 13C in soil carbonates on land, which probably reflects increased grassland too. Although records of hominin diversity before about 3 Ma are scanty, later times saw the rise of several bipedal species, grouped as the powerfully jawed parathropoids and the more daintily chewing members of the lineage that led to modern humans. Detail in those sections of marine core that were used – presumably costs prevented continuous measurements – shows that the carbon-isotopic signals in the waxes varied in harmony with evidence for climate change, so the proportions of savannah and woodland probably shifted quite rapidly. However, because cold-dry periods have tended to be longer than those which were warm and more humid, savannah would have had more influence over faunas than ephemeral woodland. Fascinating as this empirical relationship between hominin evolution and vegetation change is, what Africa lacks – as indeed does most of the planet – is data that chart accurately how topography has changed with time. Cosmogenic and U-Th/ He apatite thermochronology, on which so much hope and funding have been invested, has proved spectacularly ineffectual compared with careful work on the likely effects of changing landforms.