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To dissect evolution, Joe Thornton resurrects proteins that have been extinct for many millions of years. His findings rebut creationists and challenge polluters.
One deep-frozen vial holds the more-than-600-million-year-old ancestor of the receptors for oestrogen, cortisol and other hormones, which Thornton brought to life nine years ago. Other tubes house proteins more than 400 million years old, which Thornton resurrected a few years later to show how an ancient receptor had changed its preferences — and how the march of evolution cannot be reversed. In another corner of the freezer rest the ancient protein components of a sophisticated cellular machine that acquired a more complex form through random mutations rather than selection for superior function...
Starting with the genes for steroid hormone receptors from a slew of living organisms, he clambered backwards through the evolutionary tree to deduce the most likely sequence of the common ancestor of all such receptors, which existed some 600 million to 800 million years ago – in the common ancestor of 'you and a snail', as he puts it. Instead of stopping there, as most evolutionary biologists would have done, he then built the gene and inserted it into cells that could manufacture the ancient protein.
He chose to explore a pair of steroid hormone receptors: the mineralocorticoid receptor (MR), which binds the hormone aldosterone and regulates salt and water balance; and the closely related glucocorticoid receptor (GR), which binds cortisol and controls stress response. A gene duplication more than 450 million years ago produced the two receptors — but aldosterone didn't arise until many millions of years later. The timing seemed to make the MR a textbook example of irreducible complexity: how could selection drive the evolution of a lock (the MR) to fit a key (aldosterone) that didn't yet exist?