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Paleontologists can track some of life's transformations in fossils observing how fins gradually evolved into feet, for example. But fins and feet and other complex structures are also encoded in DNA, and until the 1980s, biologists had almost no knowledge of the genes that built them. Over the past 25 years, biologists have identified many of the genes that help build embryos. A number of them help lay out the embryo's blueprint by letting cells know where they are. The cells absorb proteins floating around them, and the signals trigger the cells to make other proteins, which in turn clamp onto certain bits of DNA to switch neighboring genes on and off. This network of genes eventually leads a cell to give rise to an arm or a brain or a tongue.
These networks are so intricate that they probably put some limits on evolution's creative potential. Once a lineage of animals evolves networks for arms and legs, it's not easy for evolution to rewire the networks to produce, say, wheels. For one thing, many networks share some of the same genes. A change to a gene that improves one network may wreck another one. So for the most part, we're stuck with what evolution gave us.
Nevertheless, new traits have evolved. Once there were no brains, and now there are billions. Once you could search the entire world and never find a leaf. Now the world is green. Biologists are discovering some of the genetic secrets for evolving new traits. One is to recycle old genes.
Growing hair, for example, is a trait that evolved only in mammals. One of the key proteins in our hair is known as alpha-keratin. Not long ago, some Austrian and Italian researchers decided to search for alpha-keratin genes in animals that lack hair. They found those genes in chickens and lizards which belong to the closest living lineages to mammals. Lizards build alpha-keratin in their claws. And it turns out that mammals do as well. The research suggests that the hairless ancestors of today's mammals already had alpha-keratin that was used to build their claws; only later was alpha-keratin borrowed to help build hair.
Darwin had no way of knowing this, since he had no way of examining DNA. If he did, he might well have rethought one of his most potent metaphors for evolution: the tree of life. It's not that the metaphor is wrong. Scientists regularly reconstruct evolutionary branches today. When a new disease breaks out, for example, the fastest way to figure out what to do is to determine what the pathogen is related to.
But there's more to the history of life than the branching of a tree. Every now and then, DNA moves between species. Viruses ferry genes from one host to another. Bacteria swap genes inside our bodies, evolving resistance to antibiotics in our own gut. Some 2 billion years ago, one of our single-celled ancestors took in an oxygen-consuming bacterium. That microbe became the thousands of tiny sacs found in each of our cells today, known as mitochondria, that let us breathe oxygen. When genes move this way, it's as if two branches of the tree of life are being grafted together.