Our branch of the evolutionary tree split from that of other apes about seven million years ago in Africa. Our ancestors had evolved to be tall and big-brained in Africa by about a million years ago. Afterward, some of those early humans spread out to Europe and Asia, evolving into Neanderthals and their cousins, the Denisovans.

Our own lineage continued to evolve into modern humans in Africa.

After decades of fossil hunting, the record of ancient human relatives remains relatively scarce in Africa in the period between 950,000 and 650,000 years ago. The new study offers a potential explanation: there just weren’t enough people to leave behind many remains, Dr. Hu said.

Brenna Henn, a geneticist at the University of California, Davis, who was not involved in the new study, said that a bottleneck was “one plausible interpretation.” But today’s genetic diversity might have been produced by a different evolutionary history, she added.

For example, humans might have diverged into separate populations then come together again. “It would be more powerful to test alternative models,” Dr. Henn said.

Dr. Hu and his colleagues propose that a global climate shift produced the population crash 930,000 years ago. They point to geological evidence that the planet became colder and drier right around the time of their proposed bottleneck. Those conditions may have made it harder for our human ancestors to find food.