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3. Environmental Change, Human Evolution and Our Movement Around the World. Rates of plate movement (cm/yr) Gamma Ray bursts; Gamma ray bursts are very short wave (high temperature) emissions usually associated with major cosmic events – supernovae and the formation of black holes. They may last from milliseconds to hours. Jet Streams and the Circumpolar Vortex The circumpolar vortex is a permanent feature of the Earth’s general circulation, but its size and behaviour vary seasonally. It expands during the winter and contracts in summer. Its edge is wave-like (Rossby Waves). The number of waves may vary as may their amplitudes. The edge is also marked by a very strong temperature and pressure gradient aloft, and because of this, a core of very strong winds – the polar front jet stream. The location of the jet stream has huge consequences for our weather. It controls air mass dominance, and directs weather systems including tornadoes. There is also a subtropical jet stream which tends to be less persistent. Jet streams are also found in the southern hemisphere, although less is known about them. Cross-section of the lower atmosphere (troposphere) The situation during much of the winter Environmental Change and Human Evolution Last week we looked at the second major control of Earth environment – solar radiation. Whereas climate appears on the scale of millions of years to be determined by plate tectonics, the finer variations , only discernible in the relatively recent past, seem to be related to both the supply and processing of solar energy. Whatever the cause, one reflection of environmental change is the fossil record. It reflects evolution and the sometime drastic response to climatic shifts. The record is really only good for the last 600,000 years, since the Cambrian. The apparently rapid diversification of life from that time is called the Cambrian Exlplosion. If we take a look at the record of biodiversity through the last 600 million years, we see that there is a logarithmic increase in species through time. However, the increase is not a smooth one. There are five episodes at which biodiversity is drastically reduced. These are called mass extinctions. The largest occurred at the end of the Permian when perhaps 90% of species went extinction. The latest one, at the Cretaceous-Tertiary (K-T) boundary, had about a 70% loss. All except the K-T extinction appear to have been caused by a complicated set of mechanisms determined by plate tectonics. At the K-T boundary we can make a good case for asteroid impact as the cause. That boundary is very significant for us. From that time we see modernization of the biosphere – the rise of mammals, insects, birds, flowering plants, etc. Evolution of animals v= birds, BB= placental mammals Evolution of plants and animals after the K-T event . angiosperms=flowering plants, ungulates=hoofed animals. One order that evolved was the Primate Order. We evolved from these as the family Hominideae, in the genus Homo. Our species? Homo sapiens. This evolutionary process appears to have been largely driven by the environmental changes from the mid-Tertiary. As climate cooled, tropical rainforests contracted and were replaced by grasslands and deserts in the tropical zone. Until the mid-Tertiary, much of the Arctic archipelago had forest cover. Metasequoia (redwood) on Axel Heiberg Island) Evolution of primates. Our ancestors, the primates, were once widely distributed, but they have always been largely tropical fauna and largely arboreal. Thus, climatic cooling caused their contraction into the current tropical zone and the loss of forest cover stimulated some to ground dwelling. This required the exploitation of different resources so there were morphological and behavioural changes. The most obvious of the latter was the development of bipedalism. Our Lineage Early primate Current distribution of primates Changes in the distribution of primates in the lemur family through the Tertiary. Red indicates fossil locations. Ring-tailed lemurs We share about 96% of our genes with gorillas And 98% with chimpanzees Our African Origins; Although climatic changes through the Tertiary were global, the evolution of hominids was not . It was confined to Africa. Was this because Africa had a suite of environmental conditions that only allowed our evolution to occur there or was it fortuitous? Before we get into the African question, let’s consider how speciation is thought to work. Speciation seems to occur when subpopulations get isolated. How does reproductive isolation occur? (a) geographic (allopatric)isolation – rivers, seas, mountains, deserts, etc. (b) ecological or temporal isolation– subpopulations develop different habitat preferences ( ecological isolation), or have different mating schedules (temporal isolation) etc. Why Africa? (a) increasing aridity and seasonality leads to major contraction of once extensive rainforest (b) expansion of tropical savanna (c) creation of isolated forest fragments in a ‘sea’ of savanna (d) isolation facilitated by uplift and volcanism in the Rift Valley system (e) high nutrient status of soils means high primary productivity (f) strong coevolutionary associations between herbivores, carnivores and scavengers. Massive environmental changes then triggered life-style and dietary changes in hominids. Obvious consequences included bipedalism and an increase in body (and brain) size. These responses are evident with both proto humans and all species of Homo. The proto humans are usually considered to be the Australopithecenes. A. afarensis , Lucy, is the best known of these. Recently, a new genus, Ardipithecus, has been recognized. It predates the Australopithecines. About 2 MYBP the first species of Homo (H. habilis) appeared, then H. erectus. Bipedalism; Bipedalism may be seen as a response the change from being arboreal to a life on the ground. Benefits – increasing mobility, increasing travel range, arms no longer required for locomotion, height gain, tool preparation, thermoregulation, etc. Changes from knuckle-walking to full bipedalism Increasing Size; Increasing size was associated with broadened dietary niche, increased home range, increased mobility, change in scavenging activities, increase in thermoregulatory efficiency, increased longevity, slower reproductive rates, increasing brain size, increasing sociality. Changes in Brain Size; Out of Africa; Humans remained African until about 1.5 MYBP. Since then there have been two pulses of emigration – the first by Homo erectus after 1.5 MYBP and the second by H. sapiens after about 100,000 YBP. Both of these exoduses involved movement northward into areas that were climatically more severe. How and why could this happen? Did our two diasporas have any obvious environmental consequences? There’s no evidence that Homo erectus did, but the coincidence of the arrival of H. sapiens and a mass extinction event suggests that the first impact of modern humans was both selective and massive – the extinction of late-Pleistocene megafauna. Was it technological? Was it a population issue? Late Pleistocene Megafaunal Extinction; 50,000 years ago all of the continents had a rich megafauna (animals larger than 44 kg), but by 10,000 BP , most large mammal genera had disappeared except in Africa. The pattern of extinction is peculiar; The continents were not impacted equally. N and S America, and Australia lost almost all of their megafauna. In Eurasia losses were less. In Africa there was little loss. The timing of extinction varies between continents. In N and S America, extinctions took place mostly between 12,000 and 10,000 YBP. In Eurasia, the extinctions started earlier, but finished at about the same time as in the Americas. In Australia, extinctions occurred early, about 50,000 to 45,000 YBP. North American Megafauna Australian Megafauna So explanations for these peculiarities need to accommodate the following questions; A. Why were extinctions confined to large animals? B. Why were extinctions in Australia large and early? C. Why were extinctions in N and S America large and late? D. Why were extinctions in Eurasia small and protracted? E. Why was there little extinction in Africa Most explanation revolve around (a) environmental change, and (b) human impact. Environmental Change – most hypotheses involve the large and massive climate changes that occurred at the Pleistocene/Holocene transition (ice age to interglacial). This may work for some places, but not for all. A strong case can be made only if there is a clear synchronicity between the time of climate change and the extinction event. Extinction of Irish Elk and Mammoth in Eurasia Human Impact – Were stone age people around in large enough numbers and with a sufficiently sophisticated technology to eliminate huge numbers of animals over a short time ? Is there some coincidence between the time of arrival of people and the extinction phase? Paleolithic hunters had a fairly sophisticated stone tool technology and strong social organization. There is evidence from kill sites of their effectiveness as hunters. Pattern of Extinction in North America Clovis Points Cave Art – Altamira, Spain Cave Art – Lascaux, France. Mammoth and mastodon remains are often found in wet sites – planning or preservation bias? Mastodon bone house – Mizhirich, Ukraine In N and S America and in Australia extinctions coincide with the arrival and spread of humans, but this is not true of Eurasia and Africa. In N and S America, the overkill or blitzkrieg hypothesis has been proposed. The lack of pronounced extinction in Eurasia and Africa and the massive extinction in N and S America and Australia has been attributed to prey naivete. The long association of people with large animals in Europe and Africa meant that those animals were wary of contact. In the other continents prey was naïve and easily eliminated. So, the first major environmental impact of humans may have been the mass extinction event in the Late Pleistocene/Early Holocene. It’s also from this time that we have the first evidence of plant and animal domestication. Domestication brought food surpluses, allowed population increase, urbanization, stratification of society – the emergence of civilization. We’ll look at plant and animal domestication and its consequences next week.