Starting with the modern human skull, we can trace our family tree back millions of years. As we travel back in time, our ancestors look less and less like us. They begin to resemble our closest relatives, African apes with much smaller brains. Our earliest ancestors were vegetarians. They did not eat meat.
Scientists believe that humans began to eat meat when the climate where they lived began to change.
There was less vegetation and the hominids were not physically suited to compete with larger animals for the remaining vegetation. They became scavengers, eating the meat of dead animals. The protein in the meat made their brains larger and caused a greater hunger for meat. About one million years ago. Getting a big Homo erectus brain though a pelvis would be impossible, so natural section favored offspring whose brains continue to grow after birth.