Why humans the only intelligent beings are on earth

Big brains seem to be pretty rare in the evolution of animals on this planet. One of the reasons for this is cost. Keeping our brain running is expensive. Just consider these facts.

750-1000ml of blood flow through the brain every minute. (3 soft drink cans worth)
In that minute the brain will consume 46cm3 (1/5 cups) of oxygen from that blood.
Your brain is about 2% of your total body weight but uses 20% of your body's energy.
The energy used by the brain is enough to light a 25 watt bulb.

In evolutionary terms this would make a big brain pretty rare. Think of any struggle between predator and prey. Say a lion and a gazelle for example. There's camouflage in their colouring so they are harder to see in long grass and there's speed and agility. The gazelle's ability to jump high. Now in this struggle the cost of either lion or gazelle to evolve a big brain is too big compared to what they'll lose in speed or agility before that brain would develop enough to make a real difference. This is true of most predator/prey relationships.

the other problem of developing a big brain is what do you use it for. You need a good way of manipulating the world for a brain to make use of it's abilities. Look at your hands, look at your thumbs. Brilliant for manipulating anything you come across. Perfect for grasping a stone or a stick to throw at or hit any approaching predator. Something to make up for what you lose running that expensive lump of grey matter.

Lots of the larger brained animals also tend to be social. Managing complex social relationships in a pack, group or family tends to increase the selective pressure on a larger brain.

So it has to be a situation in which there's a good selective pressure for a brain to grow to a large size in a body that has a good starting shape to take advantage of it.
Dolphins are pretty smart (and self aware) and so are parrots. They don't have a good body structure to really take advantage of it. A parrot can use it's beak and legs to manipulate simple odjects in a basic way, but it's nowhere near what a primate has.
Now for a creature that has a pretty big brain and decent limbs for manipulating objects other than primates look at the octopus. That is one smart animal. If humans were to become extint, that's the animal other than chimps and gorillas that might be a candidate for becoming super smart given the right selective pressures


There are other costs to a big brain. One is probably Schizophrenia (see link in the source section). It's also likely that our brain is as big as its going to get. Any further increase in size may well be disadvantagious due to the problems our brain has been left with growing to this size in the first place.





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