A single new fossil can change the way we think about human origins, but discovering it — deep in a cave or buried in rock — remains a daunting struggle for hammer-wielding paleoanthropologists.

“It can take years and luck to find the right one,” said Aurélien Mounier, a paleoanthropologist at the French National Museum of Natural History.

Now researchers like Dr. Mounier are using computers and mathematical techniques to reconstruct the appearance of fossils they have yet to find. On Tuesday, Dr. Mounier and Marta Mirazón Lahr, a paleoanthropologist at the University of Cambridge in Britain, unveiled a virtual skull belonging to the last common ancestor of all modern humans, who lived in Africa about 300,000 years ago.

The rendering of this ancestral skull, described in the journal Nature Research, is strikingly similar to fossils of about the same age found in East Africa and South Africa. The scientists propose that modern humanity arose through a merging of populations in these two regions.