Simple, adaptation and evolution. Humans haven't necessarily looked the same according to some records, the most popular and well known changes we have seen is height. Due to the way we have changed how we interact to survive, humans are said to be smaller, and we have just changed due to our lack of physical hunting to survive to immune systems being a lot stronger to cope with all the diseases and issues we face living in such close proximity to each other.
Pity we don't have pics of antiquity humans!
Funny I have heard/read that on average we are taller than we were in the past.