system
Given the following post, respond as if you're commenting on Reddit:
human
Why are humans physically weaker than our ancestors?

I've read that homo erectus were bigger and more powerful than Homo sapiens.  My understanding is that we all carry around a bit of Neanderthal DNA who were also much stronger and physically sturdy than modern man.  Why have we lost that physical advantage?  Surely strength along with more efficient brains would have helped natural selection?
gpt
There are a few different possible reasons. One is that there is often a general tradeoff between power and finesse. You can move a heavy weight, but you can't move a heavy weight very precisely. It could be that dropping overall muscle mass helped with the fine motor control we needed to make better tools. Perhaps with better control, the tools improved and we made up for the loss of power.

_*Edit: Comparison with chimpanzees and other apes give a clue here. One of the reasons other apes are so much stronger is that the tendons that attach their muscles to their bones tends to be further away from the joint, which gives more leverage and thus more overall power. But this also makes very fine movements more difficult._

Another reason is that muscle is energetically expensive. You have to eat a lot of food to build and then maintain muscle. When modern human ancestors and our very close cousins the Neanderthals lived at the same time, it was the height of the Ice Age. It is likely that food was very scarce. In that situation, it's advantageous to have less muscle mass, because you waste less energy maintaining muscle that isn't useful because there isn't anything around to kill. Instead, with less muscle, more energy can be spent on keeping the rest of your body in better repair. Or maybe there's game, but you have to chase it a long way. Modern human anatomy is _extremely_ efficient at moving long distances compared to the vast majority of other animals, and a heavier, stockier build like that of Neanderthals would be at a disadvantage. Over very long distances, like when you're chasing a migrating herd, humans may have been able to move further distances on less food, increasing the chances they survive to carry out the hunt when they catch up.

Less inherent muscle mass means it's easier to survive when you're on the brink of starvation. Neanderthals had both heavier musculature and slightly larger brains, both of which are very energy intensive to build and maintain. If it's a race to avoid starving to death, modern humans may have had an advantage. Evolution doesn't select for intelligence or strength, evolution selects for survival and reproduction. And there are situations where maximum intelligence and strength are not necessarily the most optimal strategies for reproducing.