So we have this change over time.

We have these 150 mutations that distinguish us.

Now some of these mutations are on this lineage.

But some of them are also on this lineage.

So when we're saying this 1.5 times 10 to the 8th years,

we're actually summing both of these things together.

So what we need to do is we actually need to divide by two.

So we take one point five times ten to the eighth years divided by two and

that becomes seven point five times ten to the seventh years.

Or the time to the ancestor will be 75 million years ago.

So as long ago, we can say, is roughly 75 million years ago.

Okay.

Take a second just to look that over then I'll give you one to try on your own.

So you start with this mutations per base pair per year, that was a given.

We then looked at how big a sequence we're looking at, 1000 base pairs.

We flipped this number around, basically from 1 times 10 to minus 6 to 10 to

the 6th and basically just changed it so yours was in the numerator.

And one mutation to the denominator, that made it ten to the sixth.

So for every one mutation, I have to wait ten to the sixth years.

We have 150 mutations, so

we multiply that times 10 to the 6th is where we get 1.5 times 10 to the 8th.

Okay, divide by two because mutations are rising along both lineages.

We're not looking at base differences between humans and the ancestor,

we're looking at base differences between humans and mouse lemur,

that's why we have to divide by two.

And therefore we get 75 million years.

Here's one for you to try.

Here's the time to.

What is the time to ancestor for a human to tamarin?

Well, let's assume the same mutation rate there.

Let's say in this case, you screen 10,000 base pairs of sequence.

Okay. Just so you're not using exactly

the same numbers.

Let's say you try 860 mutations.

What would the divergence time be?