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Theories of Aging
Why we get old
Simple single-celled organisms called prokaryotes, such as bacteria are the earliest forms of life on earth, and still abundant today. Much later evolved the more complex, but still single celled organisms called eukaryotes. From those humble beginnings came the multi-cellular life forms called metazoans. All animal cells, including humans, are eukaryotic cells. Since they share a common origin, they bear a resemblance to each other. Many molecular mechanisms (genes, enzymes, etc.) and biochemical pathways are conserved throughout the evolution towards more complex organisms.
Humans share approximately 98.8% of their genes with chimpanzees. This 1.2% genetic difference is enough to account for the differences between the two species. It may be even more surprising, however, to learn that organisms as far apart as yeast and humans have many genes in common. At least 20% of genes in humans that play a role in causing disease have counterparts in yeast. When scientists spliced over 400 different human genes into the yeast Saccharomyces cerevisiae, they found that a full 47% functionally replaced the yeast’s own genes.
With more complex organisms, such as the mouse, we find even greater similarities. Of over 4,000 genes studied, less than ten were found to be different between humans and mice. Of all protein-coding genes — excluding the so-called “junk” DNA — the genes of mice and humans are 85% identical. Mice and humans are highly similar at the genetic level.
Many aging related genes are conserved throughout species enabling scientists to study yeast and mice to learn important lessons for human biology. Many of the studies cited in this book involve organisms as diverse as yeast, rats, and rhesus monkeys, and all vary in the degree of their similarity to humans. Not every result necessarily applies to humans, but in most cases the results will be close enough that you can learn a great deal about aging from them. While it is ideal to have human studies, in many cases, these simply do not exist, forcing us to rely on animal studies.
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