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Humans descended from fish?

Most vertebrates, some 30,000 species of land animals including human beings, could have descended from a common ancestor that had a well-developed sixth sensory system.

india Updated: Oct 13, 2011 11:59 IST

Most vertebrates – some 30,000 species of land animals including human beings – could have descended from a common ancestor that had a well-developed sixth sensory system, according to a new study.

The common ancestor is believed to be an aquatic vertebrate, probably a predatory marine fish, with good eyesight, jaws and teeth and a lateral line system for detecting water movements.

Sharks, paddlefishes and several other aquatic vertebrates have similar sensory systems have a sixth sense, and can detect weak electrical fields in the water and use this information to detect prey, communicate and orient themselves.

According to British researchers, the vast majority of the approximately 65,000 living vertebrate species could have descended from a fish that is believed to have existed about 500 million years ago.

“This study caps questions in developmental and evolutionary biology, popularly called ‘evo-devo’, that I’ve been interested in for 35 years,” said Willy Bemis, Cornell professor of ecology and evolutionary biology at the University of Cambridge and a senior author of the paper.

Hundreds of millions of years ago, there was a major split in the evolutionary tree of vertebrates. One lineage led to the ray-finned fishes, or actinopterygians, and the other to lobe-finned fishes, or sarcopterygians; the latter gave rise to land vertebrates, he explained.

As part of changes related to terrestrial life, the lineage leading to reptiles, birds and mammals lost electrosense as well as the lateral line.

Some ray-finned fishes – including paddlefishes and sturgeons – retained these receptors in the skin of their heads.

With as many as 70,000 electroreceptors in its paddle-shaped snout and skin of the head, the North American paddlefish has the most extensive electrosensory array of any living animal, Bemis said.

Until now, it was unclear whether these organs in different groups were evolutionarily and developmentally the same.

Using the Mexican axolotl as a model to represent the evolutionary lineage leading to land animals, and paddlefish as a model for the branch leading to ray-finned fishes, the researchers found that electrosensors develop in precisely the same pattern from the same embryonic tissue in the developing skin, confirming that this is an ancient sensory system.

The researchers also found that the electrosensory organs develop immediately adjacent to the lateral line, providing compelling evidence “that these two sensory systems share a common evolutionary heritage”, said Bemis.

The study appears in the Oct. 11 issue of Nature Communications.