I have read of species of monkeys who eat meat, and not only monkeys, why do let's say a lion not evolve bigger brains as we did? They are also meat eaters.
Ok so this is a pet peeve of mine. Actually, maybe it's three.
1) The question, "Why didn't X evolve Y when Z did?" entirely misses the point of evolution. If X had evolved Y, it wouldn't be X anymore. It might even be Z. The ancestry of monkeys and humans split millions of years ago - we're the ones with the big brains.
2) In a million years, maybe lions will have evolved into something intelligent. There's probably a formal fallacy or bias to cover this situation. We are not at the end of evolution right now. It's not over. It's still going on. The selective pressure for tool using symbolic intelligence didn't occur during the past couple million years of lion evolutionary time. Maybe it will over the next million, and then Simba will be wrecking our shit in the Lion Wars.
3) What the hell does meat-eating have to do with intelligence? Sure, eating meat may have been the key to meeting the human brain's caloric needs, but to turn around and wonder why meat doesn't magically make brains larger seems to be an obvious fallacy. If I need nails to build a house, it doesn't follow that any process which requires nails will result in a house. I could just be making a pile of nails. There's a lot of coincidence required to get to human intelligence, and eating meat is only one possibly necessary condition.
Because they aren't smart enough.
Really, monkeys don't eat a lot of meat. Chimps, who eat the most, only eat 2-4% of calories as meat. They can go months without it. A lion is already optimized for meat eating. Humans need bigger brains for hunting (tracking, mental maps, tool making, planning and communicating). Fire and cooking may also have had something to do with it. If you are really interested, you should read Richard Wrangham's Catching Fire.
Why do they need to? They're well adapted to their current climate with no significant evolutionary pressure for them to change (perhaps deforestation will change this).
Eating meat doesn't make a species more intelligent. Eating meat (high calorie food more generally) ''allows'' greater brain expansion; it's not a causational factor.
We can start with maybe they haven't needed to. Most species change out of need or improved reproduction.
If monkeys or lions are thriving there's no incentive to grow bigger brains that will force trade-offs in other characteristics (such as less able to sprint through the tree-tops or smaller teeth and jaws). If they are really thriving, they may change because there are things they no longer need (body fur, claws, etc.) as the brain grows larger.
Again, I am not a scientist. Your hypothesis may vary.
As Dave S. hinted at, the answer is in Richard Wrangham's book.
Every species would benefit from being smarter. The answer to why some species or another isn't smarter does not lie in the limited benefits of being smart. It lies in the extravagant costs.
A big brain requires a lot of calories to support.The gut competes with the brain for calories. For any given caloric intake, the bigger the gut is, the smaller the brain can be. So in order to evolve a large brain, a species needs to be able to evolve a small gut. The way to evolve a small gut is to eat cooked foods, which require less digestive effort.
Humans developed a large brain when they started getting most of their calories from cooked foods. Monkeys and other species have yet to tame fire. They live on raw foods, which require a larger digestive effort, and therefore larger guts — and proportionally smaller brains.
Evolution is not a 1:1 correlation. Eating meat simply does not equate to developing a bigger brain. Other factors like developing & using tools, the development of speech, complex social systems have to be factored in. Nature is very complex. I don't need to understand every nuance of human evolution to know how to eat what is good for me. It seems to me the more I let go and become more animal-like the easier it is to be human.
Our upright posture, and the small profile it presents to the midday sun, allowed us to become the lords of the savanna during the hottest time of day. So, in effect, our legs led to the big brain. The bipedalism came first, don't yet know how or why we adopted it, but all our other weird idiosyncrasies followed after that.
Maybe you should read the actual Expensive Tissue Hypothesis. It's about calorie limits constraining selection of expensive tissue (like brain), not about meat = smart.
Just wait a few million years and see what happens. If mutations similar to what allowed humans to evolve repeat, you'll see the same results. If not, wait a few more million years.
It is after all random chance that drives the mutations, and not all are beneficial.
I was watching Nova on PBS tonight and the show What Darwin Never Knew was on. There is this hypothesis based on an observation of the human genome made by Hansell Stedman of the Univeristy of Pennsylvania School of Medicine. Specifically that an observed defect in our genome that codes for a particular type of muscle making gene limits the development of our jaw muscles--the same gene in apes is not defective. Our bite-force is just a fraction of an apes. The hypothesis is that the power of the jaw muscle limits brain growth. The muscles for chewing pull against the plates that make up the skull, the greater force causes the skull plates to fuse together after only 3-4 years of age in apes, in humans those plates may not stop growing completely until age 30. That means that in humans, brain capacity may grow into adulthood whereas for apes it stops very early.
The obvious conclusion would be that the meat-eating monkeys might be consuming a necessary element for greater brain growth but that there are other physiologic limits on the size of their skulls. So while a higher protein intake may be necessary for building larger brains, there are also physical limits involved which means that simply supplying a monkey with more protein has little or nothing to do with other selective forces that dictate whether or not an organism develops an attribute which requires more resources to maintain.