TORTOISES evolved into giants on at least seven occasions and on four continents. The finding undermines the long-standing idea that tortoises become enormous only if they are stranded on remote islands.
There are more than 40 species of , the most spectacular being the giant tortoises. On the Galapagos islands in the Pacific and Seychelles islands in the Indian Ocean, these animals can have shells more than 120 centimetres long.
These islands cover just a few thousand square kilometres. In contrast, Earth鈥檚 continents cover 150 million square kilometres. Yet they are home to just one truly large tortoise: the .
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This implies that tortoises are most likely to become huge when they live on islands, in line with a famous but controversial concept, the 鈥溾��. This states that, on islands, small animals tend to evolve larger bodies while large animals evolve to be smaller.
But tortoise biologists suspect otherwise. Fossils show giant tortoises once roamed Africa, Eurasia and the Americas, suggesting tortoises don鈥檛 need islands to evolve to be larger.
at the Museum of Paleontology Egidio Feruglio in Trelew, Argentina, and at the University of Halle-Wittenberg, Germany, wanted to clarify the history of gigantism. They drew the tortoise family tree using data from extinct and living species.
That is key, says at Tel Aviv University, Israel. 鈥淭here are two competing hypotheses about why we find giant tortoises on remote oceanic islands,鈥� he says. 鈥淭hey either got there giant, or became giant on the island.鈥� Knowing what their ancestors were like should sort that out.
鈥淲e find giant tortoises on remote islands. They either got there giant, or became giant there鈥�
The tree suggests the first giant tortoises 鈥� with shells at least 80 centimetres long 鈥� evolved 37 million years ago in Africa and Europe. They belonged to an extinct group that gave rise to continental giants at least twice more. One of those lived in North America within the past 2 million years.
All other giant tortoises belong to a second group. Its members evolved into giants on at least four occasions 鈥� including an extinct Asian species, today鈥檚 island-dwellers and the African spurred tortoise (Cladistics, ).
In other words, says Vlachos, tortoises tended to become giant on continents. He thinks the giant tortoises on remote islands today were big before they arrived, something that may have helped them get there. 鈥淟arge tortoises have more fat, so they can survive [at sea] longer,鈥� he says.
鈥淲e don鈥檛 need an island rule to explain the giant tortoises,鈥� says at the University of Antananarivo, Madagascar. Last year, her team suggested that . That鈥檚 consistent with gigantism evolving on continents.
However, with island origins seemingly ruled out by the new evolutionary tree, it is now even less clear why tortoises sometimes evolve into giants. Gigantism seems to have evolved on different continents at wildly different times.
This article appeared in print under the headline 鈥淲hen Earth was planet of the giant tortoises鈥�
