Evidence For Evolution

Everyone on Earth is related to everything that is living now and that has ever lived in the past. Fourteen billion years ago the Big Bang formed our universe and then, ten billion years later, matter came together to form planet Earth. Soon after that, the first proteins formed, possibly at thermal vents deep in the ocean. After that prokaryotes (single-celled organisms) and then eukaryotes (multicellular organisms) flourished. By the time we witness fossils of the "Cambrian Explosion," most notably from the Burgess Shale in British Columbia, there was a radiation of marine animals, many resembling modern crustaceans and insects. From those animals evolved the first animals with backbones, the chordates and the vertebrates. Lobe-finned fishes gave rise to the first tetrapods that managed to make a living on land by 385 Mya. Then ancient reptiles gave rise to dinosaurs, birds, and mammals. From some of the earliest mammals arose the group to which we belong, the primates. As Table 2.1 illustrates, natural selection slowly and unknowingly assembled humans by accumulating traits over millions of years of evolution. Charles Darwin grasped this notion well before there were many fossils to support it and well before any sort of plausible mode of inheritance of these traits was understood.

During his five-year voyage on the H.M.S. Beagle (1831-1836), Charles Darwin (1809-1882) collected evidence that he would use to construct his theory of natural selection. He gathered animals, plants, and fossils, and cultivated the ideas that he used to formulate a mechanism for evolution and natural selection. Looking back through a modern lens, the evidence Darwin observed is overwhelmingly strong today.

Table 2.1 Origins of Some Major Human Traits

Trait

Millions of Years Ago

Writing Agriculture Language Symbols

Modern brain size Meat-eating Stone tools Small canines Bipedalism Tricolor vision Dental pattern

3 ear ossicles Hair

5 digits

4 limbs Jaws

3 semicircular canals Backbone 2 eyes

23 35 130 150 340 385 460 470 520 550

0.006

0.01

Biogeography

Sailing around South America and eventually the world, Darwin witnessed species of animals and plants that very few Europeans had ever seen, many of which were unknown to European scientists. He noticed that not only did there seem to be regional similarities between organisms, but that organisms in neighboring environments tended to resemble one another rather than organisms in similar environments elsewhere in the world. In other words, organisms evolve locally.

A famous example of this biogeographic phenomenon, once noted by Darwin, is the variety of finches on the Galapagos Islands, an isolated archipelago created by volcanism and characterized by arid, rocky terrain. The finches on the Galapagos resemble the finches on the lush west coast of South America more than they resemble birds that exist in other harsh, arid environments.

Because islands lack whole groups of animals, newcomers have the opportunity to fill the open niches that are often already filled in their original environments. The original Galapagos finches competed with very few animals on these relatively isolated islands and were able to radiate into many species. They evolved a range of beak sizes and shapes, body sizes and shapes, and feeding behaviors. There are finches that behave and look like woodpeckers, but their genes link them to other finches of Galapagos, not to real woodpeckers elsewhere. Similarly in humans, it appears from genetic analyses that Melanesians with darkly pigmented skin are more closely related to Asians in that region of the world than they are to darkly pigmented Africans.

0 0

Post a comment