The new giant clam is found exclusively in very shallow waters within the easy reach of humans, which makes it much more vulnerable to overfishing than the other species, Richter said.
"The striking loss of large specimens is a smoking gun indicating overharvesting," Richter said.
That theory tracks with history, said team member Marc Kochzius, who conducted genetic analysis on the new species.
"The decline of T. costata coincided with the human coasting out of Africa," he said.
"We propose that giant clams, and especially Tridacna costata, were a valuable food resource, which was rather easy to collect on the shallow reef flat," he said in an email.
John Shea, a paleoanthropologist at New York's Stony Brook University, said the findings mesh with the hypothesis that humans began to migrate out of northeastern Africa because they were working harder for diminishing food returns.
Other factors could also have contributed to the clam species' decline, said Shea, who was not involved in the study.
"The previous decline occurred during the last interglacial period, a period of rapid climate change," where there were warmer temperatures and rising sea levels, he said in an email.
The salinity of the Red Sea could have also been a factor, he said. "It shouldn't take much change in sea chemistry to evoke major biotic changes."
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