Whosoever annoynts his feete or hands, with the grease of a Woolfe: he shall not be hurt with any colde of his handes, or feete so annointed
February 5, 2014 § Leave a comment
After genome sizes failed to fit notions of simplicity and complexity, researchers hypothesized that gene number—genes being the sections of the genome that encode proteins—might instead reflect them. For a few years, that seemed about right. Humans have about 22,000 genes while the mosquito Anopheles gambiae has about 14,000. Then, in 2007, an international team of researchers sequenced the genome of the plant-like sea anemones, marine creatures that lack muscles, heads, rear-ends, and brains. To their surprise, anemones had more genes than insects, including some genes that humans possess but flies do not. Even more perplexing: Sea anemones evolved before flies and humans, some 560 million years ago. That meant animals might have been genetically complex from the start. “When I was younger, and we knew less, we thought that organisms gained genes over millions of years and that the earliest animals were genetically very simple,” says Bill Pearson, a computational biologist at the University of Virginia who developed some of the first techniques to compare protein sequences among organisms. “We think that less now,” he adds…
It’s mutations that cause body parts to become simpler or more complex, so to see whether they naturally cause one state more often than the other, Jukka Jernvall, an evolutionary biologist at the University of Helsinki in Finland, experimented with teeth. According to the fossil record, mammals’ teeth went from tiny, pointy daggers 200 million years ago to more complex shapes with bumps and grooves. “For the first half of [mammals’] existence, teeth were pretty simple,” Jernvall says. “Then they went wild.”
Jernvall’s team induced mutations in genes involved in tooth formation in mice, and found that most of the mutations caused teeth to become simpler than they usually are. To form a more complex tooth, the team had to induce multiple molecular changes at once. The results suggest that reductions in complexity should evolve more easily than increases in complexity. Without pressure from the environment, teeth would have stayed simple. The fact that they did not means mammals with complex teeth were at a sizable advantage. Jernvall speculates that these mammals feasted on flowering plants that their pointy-toothed sisters could not grind. “Tooth complexity was ecologically driven through diet,” he says.
Jernvall’s study shows how complexity in tooth shape can evolve, but it does not speak to other trends in mammalian features, such as their number of vertebrae, changes in intelligence, or their number of genes. The sheer number of features for any given organism makes complexity an ineffable trait to grasp, says Dan McShea, an evolutionary biologist at Duke University in Durham, N.C. Shell designs and tooth bumps aren’t inherently perfect reflections of complexity, they’re just amenable to study. Furthermore, he says, people often choose to define complexity by what puts humans on top. If complexity were instead defined by features that allow an organism to survive successfully, he says cyanobacteria might be at the pinnacle level, because they have flourished for 3.5 billion years while many lineages of mammals have gone extinct within a fraction of that time. McShea warns, “This impression of directionality may be an illusion.”
Perhaps the fact that people are stunned whenever organisms become simpler says more about how the human mind organizes the world than about evolutionary processes. People are more comfortable envisioning increasing complexity through time instead of reversals or stasis. read more