The most comprehensive elephant genome study ever conducted is offering some surprises about the family tree of the world's largest land animal, while also settling a debate about Africa's elephants.

The scientists sequenced the genomes covering seven living and extinct species.

"I hope that this study can create an appreciation for the rich evolutionary history of elephants and emphasize the need for protecting the only three elephant species that still walk the planet today, who are all under imminent risk of extinction from poaching and habitat loss," said Harvard Medical School geneticist Eleftheria Palkopoulou, one of the researchers.

The research found multiple instances of gene flow - interbreeding - between different extinct elephant species, though this has virtually stopped among today's elephants.

The straight-tusked elephants that once inhabited Europe and Asia, the largest of the species studied at up to 4 meters and 15 tons, are a case in point. The species turns out to be a hybrid with portions of its genome arising from an ancient African elephant, the woolly mammoth and the African forest elephants still alive today.

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Straight-tusked elephants were traditionally thought to be most closely related to Asian elephants due to similarities in their skulls and teeth. One of the two straight-tusked elephants studied lived 120,000 years ago and provided one of the oldest high-quality genomes for any extinct species.

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The scientists also found fresh evidence of interbreeding among the Ice Age Columbian and woolly mammoths, which crossed paths in locations where the more temperate regions of North America met the glaciers that then covered large parts of the continent.