Researchers found the world’s oldest animal sperm in a 50-million-year old fossil of an unidentified worm species by complete accident.
Scientists from the Swedish Museum of Natural History in Stockholm reported that they had found some long, slim cells within the worm fossil while trying to analyze the cocoon with an electron microscope.
The group noted that the cells are the oldest traces of animal sperm ever found on our planet. The fossil was unearthed by Argentinean researchers on a island off the Antarctic Peninsula.
A paper on the discovery was published July 15 in the journal Biology Letters.
Because of their fragility, animal sperm cells are not a common appearance, unlike plant sperm cells that are more resistant. The latest traces of plant sperm were deemed 400-million year-old.
Yet, the findings are not unique. Another team found animal sperm traces in a Baltic amber. The 40-million-year-old cells belonged to several springtails, researchers said.
Benjamin Bomfleur, lead author of the worm sperm study, recalled that he and his team laughed when they realized what they were looking at with the microscope.
“[…] but in retrospect, it makes sense that you would find them as common inclusions in fossil cocoons,”
Cocoons are common deposits used by worms and leeches alike to store eggs and sperm inside. As the embryos develop, the outer layer of the cocoons becomes more rigid to protect them.
The team has yet to find the type of worm the sperm cells belong to. Scientists can only speculate that the structure of the cells may indicate that the material belongs to a crayfish worm, which routinely lives on lobsters, too.
On the other hand, researchers not involved in the study argued that those worms dwell only in the northern hemisphere so finding them in Antarctica would be at least surprising. Bomfleur responded that what his team found may be an ancestor of the crayfish worm, which had similar sperm cells.
Researchers noted that they may never find whether the two species of worms are remotely related because the DNA in sperm cells is too damaged after such long time. The team, though, hopes that the newly found cells may not be just an outer shell of the initial cells, but they would also keep their inner microstructure.
If the microstructure is intact, scientists may have a chance of comparing it with that of other worm species and tell whether the species are related.
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