New research suggests that the animals evolved from mammals like the sea cow which is still found in some of the world's oceans.

Ann Gaeth: " Elephants are weird animals"

The elephants' aquatic ancestry was revealed by the careful laboratory study of one embryo and six foetuses, ranging in age from 58 to 166 days old.

Ann Gaeth, at the University of Melbourne, Australia, had the rare opportunity to look at the foetuses after her colleague, Roger Short, was sent the specimens from the Kruger National Park in South Africa. The foetuses were taken from elephants killed in a culling operation in 1993.

Ancestral features

She found that all the elephant foetuses contained a physiological curiosity called a nephrostome. This is a funnel-shaped kidney duct found only in freshwater fish, frogs and egg-laying reptiles and mammals.

Elephants were once at home in the ocean

"The elephant is so unusual," Ms Gaeth told BBC News Online. "No other mammal that produces live offspring has these nephrostomes."

The nephrostomes appear very early in embryo development and then disappear. "Something that appears early in gestation is much more likely to be ancestral. Those features that appear later in development are likely to be related to more recent adaptations," says Ms Gaeth.

Ms Gaeth says that fossil evidence indicates that elephants left behind their aquatic life about 30m years ago. She has identified changes to their bodies that have occurred to adapt them to life on land.

Up periscopes

Their lungs now allow the elephants to suck up a large amount of water in their trunks and hold it there, before letting it gush into their mouths.

The youngest embryo studied was 58 days old and 4mm long

The trunks themselves appear extremely early on in foetal development, being shown in even the earliest embryo examined. This suggests that they were used when the elephants' ancestors lived in water, probably as snorkels.

Modern elephants still use their trunks in this way. When Asian elephants used for logging are required to travel from one island to another, they frequently swim.

Their necks are too short to allow them to breathe with their mouths, so the trunk is pushed up like a periscope and used as a snorkel.

Internal testicles

Further embryonic evidence that elephants once swam is that, unlike other land-living mammals, they have internal testicles and always have done. Seals and whales also have internal testicles, but only acquired them when their land-living ancestors took to the seas 60m years ago.

Fossil studies of elephant ancestry have been supplemented in recent years by DNA, biochemical and immunological evidence, all of which show that aquatic beginnings were likely. The modern elephants' nearest relatives are the sea cows - dugongs and manatees.