麻豆传媒

BY THE YARDSTICK of a human lifetime, time itself is dizzyingly long. If you condensed the entire 4.5 billion years of Earth鈥檚 existence into a 24-hour period, more than 3 million years would pass each minute. The dinosaurs would go extinct at 11.39 pm and 48 seconds. Human history would begin two-tenths of a second before midnight.

The vast timescales at play in the universe鈥檚 past are counter-intuitive. Even when the early pioneers of geology and evolutionary biology had proved beyond doubt that Earth was very, very ancient, it was still widely believed to be no more than a few tens of centuries old. Some people still cling to that belief today.

Charles Darwin, as usual, was ahead of his time. He realised that a few thousand years were nowhere near enough to incrementally transform, say, a primitive fish into a bird of paradise or a tiny weed into a mighty oak. Aeons were in order. As , 鈥渨e see nothing of these slow changes in progress, until the hand of time has marked the long lapse of ages鈥�.

That was the dominant view for a long time, says at the University of British Columbia in Canada. Indeed, biologists fretted that the rate of evolution was too slow to account for the riot of biodiversity they observed in the world. Now, however, the problem has flipped on its head. 鈥淲e basically found over the last 30 years that evolution over small timescales is way too fast to explain patterns of diversity at longer timescales,鈥� says Pennell. 鈥淢any of the great mysteries in evolution are not why change occurs, but why it fails to occur,鈥� says .

This is known as the paradox of stasis. Viewed through the lens of deep time 鈥� a period of a million years or more 鈥� evolutionary change happens glacially, if at all. But when biologists measure short-term rates of change 鈥� also called microevolution, which happens over years to decades 鈥� it is remarkably rapid.

of body size in birds, mammals and reptiles, for example, found that they can significantly expand and shrink in the space of a few generations. But these changes don鈥檛 persist. It takes a million generations or more to evolve lasting changes, the study found. Darwin was right: natural selection is beavering away all the time, and yet evolution itself plods along like a stick-in-the-mud.

Why might that be? One possibility is evolution has accelerated over time and is now running faster than ever. That would explain why processes that can be observed over a few generations go fast, but ones requiring us to dig into the deep past went slower. This isn鈥檛 implausible, says Harmon: pervasive human influence could be accelerating evolution, and some scientists have proposed that evolution also speeds up as the total biodiversity of Earth increases. But neither would explain why microevolution runs at the same rapid rate everywhere, rather than just in areas of high human influence or biodiversity.

A more likely explanation is that evolution responds rapidly to short-term environmental perturbations. These don鈥檛 last long 鈥� and neither do the evolutionary changes they trigger. Over deep time, environments are very stable and species that are already fit, in the Darwinian sense, don鈥檛 need to change to stay this way. They only make sustained leaps when major environmental shifts demand it.

鈥淐locks can potentially function as a new kind of detector for gravitational waves鈥�

This opens up the possibility that life on Earth is more adaptable to rapid environmental change than we might give it credit for. It is undoubtedly true, says Pennell, that adaption is happening fastest where human influence is strongest, such as in cities. But that isn鈥檛 a get-out-of-jail-free card for our impact on nature. Evolution is up to the task, but it needs raw material in the form of genetic variation to work with, and that might not be available, especially with biodiversity already in steep decline. Mass extinction beckons 鈥� but it will be over in the blink of an eye. Recovery from mass extinctions usually takes several million years. That is just a few minutes on the endless showreel of deep time.