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Evolutionary Theory The Evolution of the Human Species
Landmarks in Human Evolution: Date chart 5 mya Chimpanzee and human lineages start to split 4.4 mya First hominid fossils: Ardipithecus romidus Upright stance begins and leads to larger brain 3 mya Humans moved into the open country from trees and jungles. Chimps did not do this. 2.7 to 3.1 mya Homo habilis "First family" (found by Donald Johansen) 2 mya Some Homo erectus leave Africa for Eurasia First stone tools and fully opposable thumb 100,000 years ago Modern humans appeared - ritual burials First hints of cave art 40,000 years ago Homo sapiens and development of language 25,000 years ago Last Homo erectus dies out on island of Java In his presentation Wills emphasized a number of points about human evolution:
1. It is not the case that humans suddenly emerged. Wills is a gradualist when it comes to human evolution. He does not believe there was ever a big "jump" in the evolutionary development of humans.
Who was the mitochondrial Eve? And when was she? Wills discussed briefly the search for the mitochondrial Eve, who is the archetypal ancestor of all humans. If we trace back the branches of species back to their origin point, we should be able to arrive at a single woman, the mitochondrial Eve from whom all our mitochondrial chromosomes are descended. We do not know what species she was, but she may have been Homo erectus. She was not the only woman living at the time, of course! And many of our nuclear genes are descended from genes found in other people living at the time. Peter Gogarten pointed out that the ‘Y" chromosome of "Adam" seems to date from a different time than Eve! Thus, could they have ever met? The best estimates for the mitochondrial Eve are around 200,000 years ago. Family Tree of gene changes Wills looked at the genetic heritage of humans and our closely related species, bonobos and chimpanzees. Unfortunately, we have no chimp fossil remains, because their bones decay quickly in tropical climates. With respect to overall genetic changes over time, humans have evolved just as much as orangutans over time from our respective common ancestors. The rate of DNA changes has NOT been greater in humans than in other species. The mysterious question, then, is what can account for our vast differences from our closest species? Wills noted that our DNA changes are more clock-like than our morphological changes. Our DNA and morphological changes DO NOT match the way would think that they should. This is striking. Our morphological/behavioral changes are radically different than DNA changes. What can account for this? Wills ventured that some human genes are evolving very quickly, but we just have not isolated them yet in the overall sequence of the human genome project. One of the startling results of the human genome project has been to show that humans have just a few more genes than a fruit fly. Humans have in the range of 30 to 40,000 genes, while the fruit fly Drosophilia has 26,000 genes. Wills believes that we can account for this surprising news by understanding that most DNA changes are morphologically neutral. In other words, you don’t need a big change in DNA to lead to huge shifts in morphological structure. The task of biologists now is to isolate the crucial genes that do lead to such momentous changes. Neoteny Wills discussed the subject of neoteny briefly. Neotenic development happens in humans when the rate of our childhood development is slowed significantly. Humans don’t mature as quickly as chimps do. This means that more learning can happen in humans during our development. There is more time for a richer and fuller development of our minds. William Calvin added that anthropologists have seen chimps who admire waterfalls and thus show a sense of awe and curiosity similar to traits that we supposedly reserved for neotenic humans. He also said we have not isolated a specific gene responsible for neoteny. In response to a question about neoteny, Wills noted that human curiosity may have had a role to play in the development of human consciousness. When on the African savanna 2 million years ago, humans were the only species that did not run away (fight/flight) from fires. Eventually, humans domesticated fire and were able to use its heat to stay warm and cook food as we moved out of Africa and into Eurasia. DNA level: chimps vs. humans Wills noted that a chimpanzee genome project is in the works. Soon we will be able to compare the chimp genome to ours. Strikingly, chimps have 99.9 % the same sequence of DNA as humans. But Wills pointed out that there is a common misperception in the media about this point, who often report that we have only 98.6 % of our genes in common, when in fact we share more genes than that. If a gene is 1000 bases long, and 986 of those bases are the same in humans and chimps, then we have 98.6% of that sequence in common (NOT 98.6% of our total genes). But, in spite of the small base differences (the remaining 14 bases), it is the same gene and does much the same function in both organisms. Thus, the common confusion is between counting the total gene similarity between a human and a chimp (99.9%) from the total base differences within a given gene shared by both chimps and humans (closer to 98.6%). The DNA sequence Now that we’ve mapped the human genome, we need to study which genes are turned on the most and in what regions of the body are they turned on. We have to recognize which ones are active or inactive. Proteomics is now the name of the game. How do the genes code for proteins? This is a level of magnitude more complex than the mapping of the genome. In time, we will have much more insight into the nature vs. nurture argument. What can we account for by our genes, and what can’t we? For example, to date, no one has found a gene for schizophrenia. Lastly, Wills showed some intriguing slides of a cave in Spain where 200 human skeletons accumulated over a short period of time about 300,000 years ago. The skulls range from the size of Homo erectus all the way up to small modern humans. Conclusion Overall, Wills’ presentation was a fascinating tour through the various branches of our evolutionary history. Given how we are learning that we are closely related with other species, Wills speculated that in the near future we may find that we have Neanderthal DNA in our own genome.
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