Human evolution; bipedalism and savannahs (Introduction)

by David Turell , Wednesday, September 11, 2024, 22:07 (7 days ago) @ David Turell

Savannahs did not force bipedalism:

https://www.newscientist.com/article/2447242-when-did-humans-leave-the-trees-for-the-sa...

"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.

"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 – for instance, splitting long stems along their length. The people may have been making objects like baskets, ropes and fasteners.

"The stone tools Xhauflair’s 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’t match the “rainforests bad for humans” scenario. In fact, it fits an emerging body of evidence that humans and our hominin relatives often lived in dense forests.

"Let’s first consider the recent past, by which I mean the past 10,000 years or so – 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...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 “ghost cities” suggest that complex societies didn’t all develop in the same way: the Amazon inhabitants lived in built-up areas (made from mud, not stone), but they didn’t chop down the forest or fully convert to a farming lifestyle.

"Put simply, these people didn’t leave the trees behind.

***

"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.

"...The “hobbits” (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 at least 134,000 years ago.

***

" The classic notion, which emerged gradually in the 20th century, is called the savannah hypothesis. It’s 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: “The famous ‘savannah hypothesis’ is no longer tenable.” 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.

***

"Domínguez-Rodrigo’s 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 “is no longer tenable”, but that there is “compelling support” for the mixed-habitat hypothesis.

"Further support came in 2020, with a study that tried to reconstruct how African habitats have changed over time 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 – which would have pushed apes and hominins out onto the grasslands. On the other hand, the timings don’t really match. Remember, we don’t 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’t 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.

***

"Hence my suspicion that the core idea of human evolution in the 20th century is, if not entirely wrong, seriously incomplete. The story isn’t 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."

Comment: the savannah theory skips the point that things happen slowly over time. There is no sudden appearance of vast grasslands forcing an immediate upright posture.