The great apes
Humans are part of the family of primates known as hominids. The other hominids, or great apes, are orangutans, gorillas, bonobos and chimpanzees (our closest relatives). We are the tailless primates. Of this small group of mammals, humans are the only ones who do not sleep in trees. Why is this, and what does it mean for us as a species? The answer may interest you.
Fire
Homo Sapiens, or modern humans, evolved from Homo Erectus. Scientists believe that it was Homo Erectus who first used fire in a controlled way, somewhere between 1.7 and 2 million years ago. Having previously slept in trees, the introduction of fire meant that these people could move to ground sleeping. Keeping a fire burning at night meant that they stayed warmer and it also kept predators and insects at bay. This move to ground sleeping had enormous significance for the rapid development of the brain.
REM sleep
During sleep, there is the deep, recuperative stage and there is the dreaming stage. When we are dreaming – in REM sleep – our brains are processing and programming, laying down memories, making connections and, in general, developing understanding and intelligence. Our dreams can be vivid and full of action. Remember, our subconscious does not know the difference between what is real and what is imagined so, as far as it knows, everything that is happening in our dreams is happening in real life.
Sleep paralysis
The expected response of the brain to the events in dreams would be to send messages to our bodies to actually do the things that we are dreaming about. Clearly this has safety implications. If we were to act out all we dream of, we would put ourselves in great danger. To combat this, the body has a marvellous way to keep us safe: sleep paralysis. When we are in REM sleep, our bodies are paralysed and this stops us from moving around and doing crazy things.
Now, as tree sleepers, sleep paralysis presented another danger. Imagine a great big heavy weight of a person, asleep in a tree, paralysed and unconscious. Sounds precarious. Therefore, when we slept in trees, our levels of REM sleep were kept to a minimum – enough to process what was necessary and it stopped there. When our ancestors began sleeping on the ground, this problem was eliminated and we evolved to have longer periods of REM – processing – sleep.
The development of the human brain
What this led to was a period of rapid brain development which built upon itself. As Homo Erectus’ brain developed, the REM sleep had ever-more complexity to work with meaning that richly connected synapses advanced exponentially. It wasn’t just individuals who benefitted. This brain growth meant that communication became more sophisticated. Humans were able to share ideas and teach one another. Art, music and creativity were expounded. Societies grew.
There is no doubt that humans are winning at the evolutionary game. We are highly intelligent and we exist in elaborate social structures. It can be argued that it was the move from tree sleeping to ground sleeping that enabled humans to become the powerful species we are today.
Rebecca Bartholomew, 27 June 2022
source: Why We Sleep by Matthew Walker
