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Human Evolution
Wednesday 01 April 2015

Human Evolution - most evolutionists believe that climate is a motor of evolutionary change.

This week we are looking at some of the ideas explored in The Songlines by Bruce Chatwin. Here we consider 'jumpers', climate and 'success'. For this we are featuring the work of Hugh Brown, professional photographer based in Perth, Western Australia.

Hugh Brown photographer

Orthodox Darwinians believe that evolution proceeds in a stately continuum. Each generation will differ imperceptibly from its parents; and when the differences are compounded, the species crosses a genetic 'watershed' and a new creature, worthy of a new Linnaean name, comes into being.

The 'jumpers', on the other hand - in keeping with the brutal transitions of the twentieth century - insist that each species is an entity with an abrupt origin and an abrupt end; and that evolution proceeds in short bursts of turmoil followed by long periods of idleness.

Species, on the whole, are conservative and resistant to change. They will go on and on, like partners in a shaky marriage, making minor adjustments here and there, until they reach a bursting point beyond which they cannot cope.

In a climatic catastrophe, with its habitat fragmenting all around, a small breeding community may get hived off from its fellows and stranded in an isolated pocket, usually at the far end of its ancestral range, where it must transform itself or die out.

The 'jump' from one species to the next, when it does come, comes quickly and cleanly. Suddenly, the new arrivals no longer reply to old mating calls. In fact, once these 'isolating mechanisms' take hold, there can be no genetic backsliding, no loss of new features, no going back.

Sometimes the new species, invigorated by the change, may re-colonise its former haunts, and replace its predecessors.

The process of 'jumping' in isolation has been called 'allopatric speciation' ('in another country') and will explain why, whereas biologists find countless variations within a species - in body size or pigmentation - no one has ever found an intermediate form between one species and the next.

The really important news is that we belong to a most stable lineage.

The ancestors of man were 'generalists': resilient and re-sourceful creatures who, over the same period as the impala, will have had to wriggle out of many a tight corner without having to speciate at every turn. It follows that when you do find a major architectural change in the hominid line, there must be some ferocious external pressure to account for it. Also, that we may have a far more rigid moral, instinctive backbone than we hitherto suspected.

Since the close of the Miocene, there have, in fact, been only two such major 'leaps forward', separated one from the other by an interval of roughly four million years: the first associated with Australopithecus, the second with man:

1    The restructuring of the pelvis and foot from those of a brachiating forest ape to those of a walker on the plain; from a four-limb to a two-limb plan; from a creature that moved with its hands to one whose hands were free for other things.

2    The rapid expansion of the brain.

Both 'leaps', it turns out, coincide with sudden shifts towards a colder and drier climate.

Around ten million years ago, our hypothetical ancestor, the Miocene ape, will have spent his days in the high-canopy rain forest which covered most of Africa at the time.

Like the chimpanzee and gorilla, he will probably have spent each night in a different spot, yet confined his wanderings to a few unadventurous square miles of territory, where food was always available, where the rain fell in runnels down the tree-trunks and sunlight then spattered the leaves; and where he could swing to safety from the 'horrors' on the forest floor.

(From Lake Ternefine in Chad I have seen the fossil skull of a Miocene hyenid: an animal the size of a bull with jaws to slice off the leg of an elephant.)

At the close of the Miocene, however, the trees began to dwindle in size. For reasons as yet unclear, the Mediterranean seems to have absorbed about 6 per cent of the world's oceanic salt. Because of the decrease in salinity, the seas around Antarctica began to ice up. The size of the ice cap doubled. The sea level fell; and the Mediterranean, cut off by a land-bridge at Gibraltar, became one vast evaporating salt-pan.

In Africa, the rain forest shrank to small pockets - where the arboreal apes are at present to be found - while over the eastern side of the continent, the vegetation became a 'mosaic savannah': of open woodland and grass country, with alternating seasons of wet and dry, plenty and want, floodwater and lakes of cracked mud. This was the 'home' of Australopithecus.

He was an animal that walked and probably carried loads: upright walking, with its development of the deltoid muscle, seems to presuppose the bearing of weights, probably food and children, from one place to another. Yet his broad shoulders, long arms, and marginally prehensile toes suggest that, in his 'archaic' form at least, he still lived partly or took refuge in the trees.

Read about the adventures undertaken by Hugh Brown as he captures timeless images: 

http://www.bradshawfoundation.com/bradshaws/hugh-brown1.php

 

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