For generations, scientists looked to the East African savanna as the birthplace of our species. But recently some researchers have put forward a different history: Homo sapiens evolved across the entire continent over the past several hundred thousand years.

If this Africa-wide theory were true, then early humans must have figured out how to live in many environments beyond grasslands. A study published Wednesday shows that as early as 150,000 years ago, some of them lived deep in a West African rainforest.

“What we’re seeing is that, from a very early stage, ecological diversification is at the heart of our species,” said Eleanor Scerri, an evolutionary archaeologist at the Max Planck Institute of Geoanthropology in Jena, Germany, and an author of the study.

In the 20th century, after scientists found many fossils and stone tools in East African savannas, many researchers concluded that our species was especially adapted to life in grasslands and open woodlands, where humans could hunt great herds of mammals.

Only much later, the theory went, did our species become versatile enough to survive in tougher environments. Tropical rainforests appeared to be the toughest of them all. It can be hard to find enough food in jungles, and they offer lots of places for predators to lurk.

“You can’t see what to hunt,” Dr. Scerri said, “and you can’t see what’s coming for you.”

But in 2018, Dr. Scerri and her colleagues challenged the idea that East African grasslands were the single cradle of humanity. The abundance of stone tools and fossils found there, they argued, might have meant simply that the region had the right conditions for preserving those traces of history.

The scientists pointed to other fossils and stone tools discovered from southern and northern Africa. Those artifacts had often been dismissed as the products of extinct human relatives, rather than our own species.

Dr. Scerri and her colleagues suggested that for hundreds of thousands of years, our forerunners lived in isolated populations across Africa, periodically mixing their DNA when they came into contact.

If that were true, then early humans should have also been present in West and Central Africa, where rainforests were common. The oldest firm evidence of humans in African rainforests dated back just 18,000 years. But the acidic soils in tropical forests could have destroyed the bones before they turned to fossils, and tools could have been washed away.

Dr. Scerri came across an older report about a site in the Ivory Coast. The researchers dug a massive trench in a hillside called Anyama. In the hard, sandy sediment, they discovered bits of plant matter as well as some stone tools, though they could not determine their age.

In March 2020, Dr. Scerri and her colleagues traveled to Anyama and excavated a fresh face of sediment, where they found more stone tools. But they worked for only a few days before the Covid pandemic forced them home. They returned to the site in November 2021, only to discover that it had been illegally quarried for road building.

“It was absolutely heartbreaking,” said Eslem Ben Arous, a member of the team now at the National Center for Research on Human Evolution in Burgos, Spain.

Dr. Ben Arous and her colleagues discovered a small area not far from the original dig where they found more tools. But the new site has been destroyed as well.

Still, the researchers managed to gather a lot of clues. Dr. Ben Arous, an expert on geochronology, used new methods to estimate the age of the sediment layers. The oldest layer in which the researchers found stone tools formed 150,000 years ago.

The sediment also preserved wax from the surface of ancient leaves. Analyzing the chemistry of the leaf wax revealed that Anyama was a dense rainforest throughout its history. Even in the ice age, when the cool, dry climate shrank jungles across Africa, Anyama remained a tropical refuge.

Cecilia Padilla-Iglesias, an anthropologist at the University of Cambridge who was not involved in the new study, said that the work offered clear proof that people were living in those jungles — and that they were living there very early in the history of our species.

“It’s important because it confirms what other research predicted,” Dr. Padilla-Iglesias said.

Khady Niang, an archaeologist at Cheikh Anta Diop University in Senegal and an author of the study, noted that many of the oldest artifacts discovered were massive chopping tools crafted from quartz. She speculated that the Anyama people used them to dig up food or hack their way through the rainforest.

“If you move a lot, you need tools to cut the trees that hinder your path,” Dr. Niang said.

The distinctive tool kit makes Dr. Scerri suspect that the Anyama people had already lived in the rainforest long before 150,000 years ago. “They’re not people who have just arrived,” she said. “These are people who had the time to adjust to their living conditions.”