3ALB 4 HUMAN EVOLUTION, OUR GLOBAL DIASPORA AND THE RISE OF CIVILIZATION.

Transcription

1 3ALB 4 HUMAN EVOLUTION, OUR GLOBAL DIASPORA AND THE RISE OF CIVILIZATION.

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4 The record of biodiversity through the last 600 million years indicates a logarithmic increase in species through time. However, the increase is not a smooth one. It is interrupted by several mass extinction events at which biodiversity is drastically reduced the Big Five 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.

5 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 and the massive environmental changes it induced 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.

6 Evolution of plants and animals after the K-T event. angiosperms=flowering plants, ungulates=hoofed animals, macropods= marsupials.

7 The Tertiary,most of the last 65 MY, and the Quaternary, which started about 2 MY ago and continues today as the Holocene, are marked by protracted climatic cooling. The changes in climate were probably the result of changing geography the movement of Antarctica into a polar location, the closing of the Arctic basin, the formation of the central American isthmus, etc.

8 How did that cooling impact on the evolution of hominids and their movement out of Africa and around the globe? How did it shape our use of plants and animals and the development of civilization?

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11 The interaction between greenhouse gas concentration, surface albedo changes, and sea level.

12 As climate cooled, tropical rainforests contracted and were replaced by grasslands and deserts in the tropical zone, and the biomes familiar to us prairies, boreal forest, temperate broadleaved forests - formed. Until then, much of North America would have been forested. Until the mid- Tertiary, these forests extended across the Arctic archipelago!

13 Present global biomes(left) and in the Eocene (below)

14 Forests on Ellesmere Island in the Eocene.

15 The evolution of primates and hominids; It s within this climatic framework that primates evolve. That evolution reflects the removal of forests, the expansion open environments (tropical savannas) and the need to adapt to and exploit both habitats.

16 Primates were around before the K-T extinction event, but they evolved rapidly in the early Tertiary. Primates are characterized by having binocular vision and grasping hands and feet. They were initially arboreal. They evolved into lemurs, tarsiers, Old and New World monkeys, great apes and hominids.

17 Primates large and small. The earliest (left) was about the size of a mouse.

18 Our lineage

19 The term hominid is usually applied to large, bipedal primates with relatively large brains. These include us (Homo sapiens), earlier species of Homo, and the pre-homo primates, the Australopithecines (Lucy is Australopithecus afarensis) and a few earlier bipedal primates ( Ardipithecus, etc.). They date from about 6 million years ago.

20 A recent version of human evolution.

21 We share most of our DNA with primates % with our closest primate relatives.

22 Our African Origins; Although climatic changes through the Tertiary were global, the evolution of hominids/hominins 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?

23 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

24 (e) strong seasonality with fire a natural part of the cycle, and long animal migrations. (f) strong coevolutionary associations between herbivores, carnivores and scavengers.

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27 The two most important features of human evolution were bipedalism and increasing brain size and complexity.

28 Bipedalism; Bipedalism may be seen as a response to changing lifestyle; 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.

29 Increasing body and brain size; Increasing size was associated with broadened dietary niche, increased home range, increased mobility, change in scavenging activities, increased longevity, slower reproductive rates, increasing brain size and complexity, increasing sociality.

30 Humans appear to be confined to Africa until the exodus of Homo erectus about 1.7 million years ago. In the light of recent discoveries, what happened after that is becoming more murky than it was.

31 Our global diaspora. Two main models have been proposed to explain the spread of humans around the globe; 1. The Out of Africa Model by which human evolved in Africa then spread to other parts of the world (twice!). 2. The Multiregional Model initial spread by H. erectus, then regional evolution of various strains of H. sapiens.

32 Both models propose an initial migration by Homo erectus around 1.7 million years ago, but they differ with what ensues. The Out of Africa model is the most popular, but on the basis of recent discoveries, it appears to be far too simple and may also require rethinking of H. sapiens origins.

33 Homo erectus and the first Out of Africa exodus.

34 H. sapiens already Out of Africa?

35 A second exodus and interbreeding with H. erectus?

36 The later migrations

37 So modern humans evolved earlier than previously thought. Where is now not obvious. They spread probably at least 100,000 years earlier than previous estimates. There were probably several exoduses. They interbred with species evolved from the much earlier Homo erectus movement out of Africa.

38 How did climate influence the diaspora? Glacial advances shaped our movement by physically blocking movement into some regions. Ice impeded movement in Europe and determined dispersal routes in North America. At full glacials, sea level was 120 m below current levels. The resulting land connections facilitated dispersal from Africa, across the Near East, through southeast Asia to Australia and from Siberia into Alaska (Beringia).

39 Sea level at maximum glaciation 120 m below current sea level.

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41 The last glacial/interglacial transition and human impact; The current interglacial, the Holocene, is a period marked by increasing human impact. Two big changes occurred at the transition from the last glaciation around 12,500 years ago; A. the massive extinction of large animals, the megafauna, and B. the beginnings of plant and animal domestication.

42 A. Late Pleistocene megafaunal extinctions The early human migrations did not seem to cause any obvious environmental impacts, but the later ones (to Australia and the Americas ) are marked by a massive die off of large animals (megafauna). In North and South America, 75-80% of big animals became extinct. In Australia, over 80%.

43 North American Megafauna

44 Australian Megafauna

45 Were these extinction caused by over-hunting, by climate change or by both?

46 B. Plant and animal domestication; The early Holocene is marked by a shift from hunting and gathering to plant and animal husbandry. The changes were incremental, not instantaneous. It occurred in several widely separated places at about the same time.

47 What was domesticated? Where and when did it happen? How and why did it happen?

48 What was domesticated? For plants, it was things that grew rapidly, required little attention and which were at hand plants of disturbed places (weeds). The first domesticates were cereals. Then legumes. For animals, it was those that lived close to humans, were gregarious and unaggresive (i.e. easily herded) and species that bred easily, grew quickly and were multi-use sheep, goats, cattle, etc.

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50 The continued importance of cereals in food production.

51 Only a handful of animal species have been domesticated. Most lack the favourable behavioural characteristics.

52 Where and when? Broadly concurrent in several unconnected places. Most of those are dry today, but at the start of the Holocene climate was wetter. What comes from where?

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54 How and why? A need to produce more food? A need for food security? A shift in climate made it essential? It was a natural extension/consequence of an already intimate relationship (coevolution of plants, animals and people)?

55 The Younger Dryas and the last glacial/interglacial transition. Did climatic instability drive people to farming?

56 Consequences Creation of a food surplus. Change from nomadism to sedentism. Urbanization. Division of labour and the development of trades. Government structures, public buildings, etc. Development of organized religion. Standing armies.

57 In short, plant and animal domestication gave us civilization. It also gave us trade, competition for resources and warfare, the subjugation of women, the emergence of epidemic disease, and the early and rapid degradation of environment.

58 Next week we ll take a quick look at the rise and fall of early civilizations and how climate may have been largely responsible. One major driver has been El Nino. It has impacted most obviously on societies in Central and South America, but has global effects, particularly in monsoonal regions.