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American Association for the Advancement of Science

Lizards' lessons for life

A juvenile Anolis ricordii from Río Limpio, Dominican Republic. This species of giant anole, which lives in the tops of trees, is convergent with counterparts on other Greater Antillean islands.
[Image courtesy of Miguel Landestoy]

If you could somehow hit "replay" on the history of life, what would happen? Would the animals we see in this Replay version of Earth look the same as the animals we see now?

Scientists have been talking about this idea for years. Some of them think that the Real Earth and the Replay Earth would look very different. They argue that too many things in Earth's history happened only once—and sometimes by chance. Those things shaped the way life looks on Earth today, they say, but there's no reason to think that they would happen again on the Replay Earth.

But, other scientists think that history might repeat itself if we could build a Replay Earth. These scientists say that, when animals and plants change over millions of years to fit in better with their surroundings, they usually change in some predictable ways. So Replay Earth might look a lot more like Real Earth than you might expect.

Luke Mahler at the University of California, Davis and his fellow scientists found a way to test out what might happen on this Replay Earth, in a study published in the 19 July issue of Science. They decided to look at lizards in the Caribbean islands. It took 40 million years for these anole lizards to spread out to all of the islands. And each time the lizards came to a new island, it was kind of like hitting the replay button on lizard life.

So, what did the scientists find out from the replay reptiles? It turns out that the lizards on each island divided up into different species in some predictable ways. Each island had lizards that were experts at living on twigs, and each island had lizards that loved living on the ground. The scientists think that there must be a few great ways to live as an anole lizard, which these Caribbean lizards discovered each time they hit a new island.

Mahler and the other researchers say that their experiment shows that there are some common patterns that happen every time life evolves—so maybe the Replay Earth might not look so different from our own.