Some of our ancestors ate meat. It consisted of 25% or less of their diet.
We still don't have any adaptations for it.
This is a good lecture covering this: http://www.youtube.com/watch?v=h0PF5R0ywp4
Another article: http://www.takepart.com/article/2013/07/16/science-debunks-paleo-diet-again?cmpid=tp-ptnr-nutritionfacts
But, so what?
Why do you think it matters morally whether our ancestors ate meat or not??
We can thrive on a vegan diet.
Harming and killing animals for "pleasure"/convenience isn't moral. Thus veganism is our moral obligation.
I highly recommend you to study this article: http://www.abolitionistapproach.com/what-michael-vick-taught-us/#.UfTj1m37aQI