What are the odds?

Great Debates
1

that someone was born in the past recorded history or before that, that looked just like you or me?
I guess I am asking just how many variations of the human genetic code do you think there are and if it happens to be a finite number then how many times does it have to go around before it repeats itself? Or would you say that the human genetic code is infinite and that no one has ever been born that looked exactly like you or me?

Of course the rest of the creatures on this earth seem to look pretty much alike…ie whales, deer, turtles…so I was wondering if it is possible that in the last few hundred thousand years that someone was born with my exact or near exact genetic code and looked just like me?
Or would you say that of the billions of people that have been born since time began each was totally unique and has* never* been duplicated in any way?

2

Um… twins?

But seriously, with dominant genes through certain bloodlines I think it is very possible. There are often people who look a lot like their great-grandmother or something.

As far as exact genetic code, fairly doubtful. Only a small percentage makes up what you “look like” though, so there is a good chance there.

3

I’d guess that millions of butt-ugly people have been born in the past few hundred thousand years. What’s your point?

4

Well, I’m not an expert, but the numbers I’ve got look like this:

There are between 2.8 and 3.5 million base pairs in a humans DNA. Each base pair effectively represents 2 bits, since it can have 4 states. I’ll use the 3.5M number.

Approximately 97% of that data is believed to be random junk.

Therefore, there are 210,000 bits of definition to a human. A totally random generation of those bits gives you 2^210,000 possibilities or about 10^63,000. That’s a lot.

So, if each human is a random jumble of those bits, the odds are 1/10^63k that a given human will be exactly like you, i.e. virtually impossible. To put it in perspective, only ~100B humans have ever lived.

Now, the sharp observer will note that each human is not a random jumble of those bits, that certain pieces are extremely common, others extremely rare, and the vast majority of the possibilities would not result in viable humans (or even humans period). So, that 10^63k number above is way high. But even so, it’s such a big number that I think “never” is a fair answer.

On the animals, I suspect that the typical deer or penguin thinks that all humans look alike, black, white, blonde, redhead, male, female, young, old. But if you’re a deer or a penguin, you can tell differences between all the other deer or penguins.

5

Approximately 3720 to 1.

Well, okay, seriously…

What is “looking exactly like…?” There are really only five important features to a person’s face: forehead, eyes, nose, mouth. It’s entirely possible for hundreds of people to share very similar features like those, and therefore, to the casual glance, look “exactly like…”

But from a perfectionists (read: “anal” :D) point of view, you’d need the EXACT shape of the eyes, EXACT iris design, EXACT brow curves, EXACT folds of skin… etc.

So what are the odds of someone having looked “exactly like” you? Astronomically slim. What are the odds of someone having looked “eerily similar” to you? Not too bad at all.

6

As usual, it all depends on your definitions. How close is close enough? The more you expand your definition, the more people will fall in it. If you contract it enough, you’ll get a set of zero.

BTW, there is, of course, a lot more to it than genetics. Go find a set of identical twins and ask if people can tell them apart. A lot of their parents have no problem.

Some other factors to conisder are:
Diet (which has changed over history)
Amount of exercise (ditto)
Amount of exposure to sunlight (ditto)
Environment (ditto)
Personal history - scars, etc.
Random chance (remember that genes merely code for proteins, which act on the molecules that are lying around. There is an element of chance in every reaction in the body)

So not only would someone have to have the same genes, which I think is pretty unlikely (remember evolution? Genes change a lot over time), but they would have had to have led a similar life in similar circumstances. Of course, like I said, none of that may matter, depending on how you’re defining “looked just like.” I’m going with "close enough that we couldn’t be told apart.

7

I’m Chinese…we all look alike!!

8

Humpty Dumpty, you will recall, told Alice that all people looked alike, and it was impossible to tell them apart because of that. Two eyes, nose in middle, mouth underneath…

9

I have met two people, unrelated to me or to each other, who looked so much like me that people I had worked with for a year would get us mixed up.

Now all I have to do is count up how many people I have ever seen, and I can give you the odds.

Russell (the real one)

10

> that someone was born in the past recorded history or before that, that looked just like you or me?

A lot of people have lookalikes, just not in an identical DNA way. We used to play a game at theme parks- searching for lookalikes of celebrities or people we knew. It’s not hard to find them.