A study of howler monkeys has revealed that different species interbreed much more than thought - implying that early species of human may have done the same.
But the findings aren't accepted by everybody - and the fossil record isn't much help in clearing the matter up.
Now, a University of Michigan-led study of interbreeding between two species of modern-day howler monkeys in Mexico is helping explain why fossil remains alone aren't enough to reveal the history of human hybridization.
The two species, mantled howler monkeys and black howler monkeys, diverged about three million years ago. They're different from one another in many ways, including behavior, appearance and even the number of chromosomes they possess. They have quite separate geographical distributions, except for the state of Tabasco, where they coexist and interbreed.
And the genetic analysis shows that it's impossible to tell individuals of mixed ancestry who share most of their genome with one of the two species from pure-bred individuals of that species. In other words, species can interbreed without leaving any fossil evidence.
"The implications of these results are that physical features are not always reliable for identifying individuals of hybrid ancestry," says evolutionary biologist Liliana Cortés-Ortiz of U-M.
"Therefore, it is possible that hybridization has been underestimated in the human fossil record."