Start with a postcard sized photo of your dad. Then one of his dad, and his dad, and keep going back for 185 million generations. How long would the shelf holding those millions of postcards be? It’d be about 40 miles long. Your picture would be at the 40 mile mark and your ancestor’s from 185 million generations ago is at the other end, at mile zero. That picture at mile zero would be of a fish!
There isn’t a distinct point when you could say a fish becomes a man. The natural selection process is slow, but steady, with lots of hard-to-notice small changes.
If you looked back 400 generations, about 10,000 years, your ancestor would look like we do now if you dressed them up. You could have kids with someone living around them. But go back 50,000 generations, about a million years, and your ancestor will be different enough to be called homo erectus, instead of sapien, and you wouldn’t be able have kids together, if you were even interested in trying to do that.
So, at what point could you call our ancestral fish’s offspring a man? That depends on how tightly you define man. Compare it to when you were growing up. When was the exact time when you the toddler, became you the child? The distinction is sort of subjective and hard to say exactly. But the thread of you is there. As time goes on you’ll look similar but just a little different, compared to pictures nearby. Each postcard picture will be the same species as relatives on the postcards on either side of the one you’re looking at.
So it’s hard to say when humans became humans. But it’s fun thinking about the long thread that is you and how long you’d be recognizable.
I need to give credit for this story idea to Richard Dawkin’s “The Magic of Reality.”