The idea that everyone is born with the same fitness level is rather crazy to me. For one thing there's things like fetal alcohol syndrome, malnourished mothers, radiation damage before birth. But much more than that there are recognized genetic disorders, which at the extremes nobody would say confer the same fitness - down's syndrome etc.

Is there any reason to believe that genetic issues obey a distribution where they are either very extreme, or have no effect at all? i.e. if everyone not classifiable as having a really extreme disease is equal? I don't think so.

I think it's much more likely that there is a normal distribution of breakdowns in fitness - a few very severe problems which manage to survive to birth, then many, many more variants with negative fitness, or ones which are good but very picky, and which may be left over from environments where they were +fitness. Then there are probably variants which are all-around good.

So the idea that we're all born with the same fitness level is a pretty extreme example of invalid "blank slate" type thinking.