May 21, 2007 — Four hundred and fifty million years ago, sharks and humans shared a common ancestor, making sharks our distant cousins. According to recent research, this kinship is evident in our DNA, since at least one shark species possesses several genes that are nearly identical to those in humans.
The elephant shark's genome is so similar to ours that we wind up having more in common with it — genetically speaking — than with other species, such as teleost (bony skeleton) fishes, which are nearer to us on the evolutionary tree.
"This was a surprising finding, since teleost fish and humans are more closely related than the elephant shark is to humans," lead author Byrappa Venkatesh told Discovery News.
Venkatesh, principal investigator at the Institute of Molecular and Cell Biology in Singapore, and his team determined that sets of genes on chromosomes, as well as actual genetic sequences, are "highly similar in the elephant shark and human genomes."
The researchers not only analyzed the elephant shark genome, but also the genes for other animals including pufferfishes, chickens, mice and dogs.
Their findings were recently published in the journal PloS Biology.
The researchers identified 154 genes in humans that have comparable matches in mice, dogs and elephant sharks. The similarities between people, mice and dogs were not unexpected, given that they are all mammals. Sharks, however, are cartilaginous fish that seem to bear little physical resemblance to mammals.
Upon closer examination, sharks and humans do share certain physiological and biochemical processes.
One is sex.
"A common feature between the elephant sharks, other sharks and humans is that in all, the fertilization occurs internally, whereas in teleost fishes, fertilization occurs externally," Venkatesh explained.
Many of the comparable genes between elephant sharks and humans are involved in the production of sperm. Both species produce sperm that appears to have receptors on the tip that allow fusion with a female egg.
Bony fish do not have such receptors. Their sperm instead enters eggs through a pore called a micropyle, which sharks and humans lack.
The researchers also found that shark and human immune systems are very similar, since sharks have all four types white blood cells found in mammals.
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