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It鈥檚 a truism in human evolution that we came down from the trees and out into more open country like grassy savannahs. The open grasslands are supposed to be more favourable habitats for hominins like us. In contrast, dense tropical forests have been thought of as 鈥渉ostile, unfavourable frontiers鈥� that were 鈥渢oo hostile for humans throughout much of prehistory鈥� ().

You know I鈥檓 only establishing this totemic idea to knock it down, right?

Let鈥檚 start with Tabon cave, on the island of Palawan in the south-western Philippines. It鈥檚 one of a complex of more than 200 caves that honeycomb a promontory on the coast. Tabon cave has been excavated on and off since the 1960s and was the first place where prehistoric people were identified in the Philippines. Our species lived there at least 40,000 years ago.

Researchers led by at the University of the Philippines Diliman in Quezon City have been re-examining the evidence from Tabon cave to find out more about the people that lived there. .

Xhauflair鈥檚 team examined the vast amounts of bat and bird guano in the cave. These animals ate insects, which in turn ate the plants in the surrounding area. The carbon isotopes in the guano were characteristic of forest plants, not of grasses, implying that the environment was a tropical rainforest. In line with this, the team found pollen from trees like she-oak and mangroves.

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The researchers also looked at the stone tools found in Tabon cave. Of 41 studied in detail, 23 had signs of being used to cut and prepare plants. Some of them seem to have been used for cutting hard plants like bamboo 鈥� for instance, splitting long stems along their length. The people may have been making objects like baskets, ropes and fasteners.

The stone tools Xhauflair鈥檚 team studied were found in layers dating from between 39,000 and 30,000 years ago. This is long before agriculture, so the people using them were hunter-gatherers. Clearly, they succeeded in living in a tropical rainforest for thousands of years.

This doesn鈥檛 match the 鈥渞ainforests bad for humans鈥� scenario. In fact, it fits an emerging body of evidence that humans and our hominin relatives often lived in dense forests.

Life in the trees

Let鈥檚 first consider the recent past, by which I mean the past 10,000 years or so 鈥� the period in which agriculture became more widespread.

Some of the strongest evidence for people living in tropical forests in this period comes from the Amazon. Over the past few millennia, this vast rainforest was filled with sprawling settlements and the inhabitants cultivated dozens of plants and animals. Just last year, researchers estimated that there are more than 10,000 undiscovered archaeological sites in the Amazon. These 鈥�ghost cities鈥� suggest that complex societies didn鈥檛 all develop in the same way: the Amazon inhabitants lived in built-up areas (made from mud, not stone), but they didn鈥檛 chop down the forest or fully convert to a farming lifestyle.

Put simply, these people didn鈥檛 leave the trees behind. Ironically, the one thing that might cause Indigenous Amazonians to leave is climate change, which could drive the Amazon past a tipping point and cause swathes of it to become savannah.

The thing is, archaeologists haven鈥檛 spent much time investigating rainforests. Partly this is because the damp conditions are bad for preserving organic remains 鈥� the evidence from the Philippines doesn鈥檛 come from a rainforest, but from a cave that was surrounded by rainforest. On top of that, stone is often in short supply so people living in rainforests tend to make artefacts from other materials 鈥� like the bamboo used by the people living in Tabon cave. In fact, Xhauflair鈥檚 study feeds into an idea called the , that prehistoric people in South-East Asia didn鈥檛 need to develop more complicated stone tools because they could make so much from bamboo.

However, in the past decade some researchers have begun exploring rainforest-adjacent sites, and have found signs of human inhabitants. The 2022 review I mentioned at the start goes through swathes of evidence, from multiple continents.

One key site is Panga ya Saidi, a cave in Kenya that contains evidence of humans 78,000 years ago living in a region with a mixture of tropical forests and grasslands. There are suggestions that the inhabitants made nets and other such perishable tools to hunt small animals.

Now let鈥檚 consider species other than our own. The 鈥渉obbits鈥� (Homo floresiensis) lived on the island of Flores, now part of Indonesia, for hundreds of thousands of years until about 50,000 years ago. Flores was densely forested, so clearly they had adapted to that environment. The similarly small Homo luzonensis from Luzon in the Philippines (not too far from Tabon cave) also inhabited a thick forest; a 2023 study concluded they did so .

Finally, let鈥檚 push further back in time, to the oldest hominins and our earliest origins.

The savannah hypothesis

Our closest living relatives spend a lot of time in and around trees. Chimpanzees, gorillas and orangutans are all heavily adapted to life in the canopy, with long arms and other features that enable them to clamber and swing through the branches.

In contrast, humans and our closest extinct relatives, like Neanderthals, are more adapted to walking upright on the ground. We can climb trees, but we鈥檙e not as good at it as chimps 鈥� just as they鈥檙e not as good as us at endurance running.

This has a simple implication: during hominin evolution, we became less adapted to trees and more adapted to walking upright on the ground. We are descended from an unknown ape that lived perhaps 7 million years ago, and which is also the ancestor of modern chimps. Presumably this ancestral ape was adapted to trees.

The question is, when and why did this happen? The classic notion, which emerged gradually in the 20^th century, is called the savannah hypothesis. It鈥檚 quite simple: some of the apes left the forests and moved out onto the savannahs, and this created an evolutionary pressure to walk upright rather than the knuckle-walking apes do.

This idea has come in for a lot of criticism in recent decades. In a 2015 journal paper, Brigitte Senut wrote: 鈥�.鈥� Senut had discovered a 6-million-year-old hominin called Orrorin, which appears to have been bipedal, despite being such an early hominin and despite apparently living in a wooded environment. More recent species like Ardipithecus ramidus, from 4.4 million years ago, also walked upright and lived among trees.

(The oldest known hominin, Sahelanthropus, is the subject of a two-decades-and-counting argument about whether or not it was bipedal. Let鈥檚 not relitigate this.)

However, there are also researchers who defend the savannah hypothesis. Manuel Dom铆nguez-Rodrigo at the Complutense University of Madrid in Spain did so in a 2014 paper, titled 鈥溾�� In a fine example of Betteridge鈥檚 Law of Headlines, Dom铆nguez-Rodrigo concluded that the answer to his question was 鈥渘o鈥�.

Dom铆nguez-Rodrigo鈥檚 argument is that there are actually two versions of the savannah hypothesis: one envisions the savannahs as grasslands, the other sees them more as mosaic environments with a mix of grassland and woodland. He says the pure-grassland hypothesis 鈥渋s no longer tenable鈥�, but that there is 鈥渃ompelling support鈥� for the mixed-habitat hypothesis.

Further support came in 2020, with a study that tried to reconstruct by estimating the evolutionary ages of various savannah tree species. Between 15 and 10 million years ago, the team concluded, savannahs expanded in the tropics and subtropics, before eventually reaching southern Africa around 3 million years ago.

I am in two minds about this study. On the one hand, it does indicate that savannahs expanded while dense forests shrank 鈥� which would have pushed apes and hominins out onto the grasslands. On the other hand, the timings don鈥檛 really match. Remember, we don鈥檛 see decent evidence of bipedality until 6 million years ago: why so late, if the savannahs began expanding 9 million years earlier? Likewise, we don鈥檛 see hominins living in truly open grassland until as recently as 2 million years ago.

The answer may be that our ancestors started walking upright in trees, not on the ground. This idea is controversial but has been gathering momentum for 20 years. Early evidence came from orangutans, which can walk upright along branches, using their hands to support themselves. But orangutans aren鈥檛 our closest relatives.

However, in 2022 and her colleagues studied chimpanzees living in the mosaic habitat of Issa Valley, Tanzania. They found . In other words, bipedalism doesn鈥檛 increase with time spent on flat ground 鈥� the reverse is true. Drummond-Clarke, now at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, argues for 鈥溾��.

It鈥檚 perhaps telling that many people are deeply fond of . There鈥檚 a fancy name for this 鈥� 鈥� but the core finding is that we find being around trees relaxing.

Hence my suspicion that the core idea of human evolution in the 20^th century is, if not entirely wrong, seriously incomplete. The story isn鈥檛 how and when we left the trees. While we do spend less time in them than other apes, they are still key to our habitats and wellbeing. In a sense, we never really left.