Conversation between two apes explained

My chimp/chump story from last week is a simplified example of how one ape species split into two.  This splitting is called speciation.  The apes in the story are members of the species that was the most recent common ancestor of humans and chimpanzees (and bonobos.)

Let’s pause for one of the digressions I like to call drum solos.  Don’t think you have ever heard of bonobos?  Sure you have.  Remember that thing about a kind of chimpanzee that resolves its differences by having sex?  That’s the bonobo, but they actually are a separate species from chimps.  They split off from chimps after we did.  Thanks, Mickey

Why five million years ago?  We infer that date from several pieces of evidence.  We have 4.4 million year old fossils of creatures called australopithecines, who are our ancestors, but not chimp ancestors.  The split must have happened before then. 

DNA provides the link we have yet to find in the fossil record.  We know how quickly DNA mutates on average.  We count the differences between human DNA and chimp DNA to calculate the number of years it should take for those differences to accumulate.  The answer is five million years. 

We feel confident in the five million year figure because this method was also used to calculate the date of our split from orangutans.  The fossil record confirms that the orangutan date is correct so we know the method works.

Now that we know when the human-chimpanzee split happened, we might wonder why it happened then.  Five million years ago was the end of a harsh cold snap.  Africa was colder and drier than it had been so the forests were breaking up and turning into woodlands.  We all know about the mass extinctions at the end of the Cretaceous period.  Climate changes killed off the dinosaurs and many other animals and gave the mammals a chance.  Or when the internet liberated music and all music became free and corporate music died and only the most talented bands… OK, bad example. 

The point is that environmental changes can drive evolutionary change.  This is what happened five million years ago.  It was getting really competitive in the trees.  Those who were above average at tree living were more likely to survive to reproduce.  Those who could make it elsewhere were also more likely to reproduce.  Those who reproduce are likely to pass along whatever advantages they have to their children.  The advantages don’t have to be very big to have a large effect.  Even small advantages will become more common over a number of generations. Future mutations will increase existing advantages or create new ones.  The mutations eventually add up to a huge change. It’s important to remember that this takes place over a very long time.  It’s not like in the Ringo Starr movie Caveman in which people go from walking bent-over to walking fully upright in a minute. 

If, as in this story, the proto-chimps stay in the trees and the proto-humans live on the ground, the two groups will stop inter-breeding.  Eventually, so many differences will accumulate that the two groups can no longer inter-breed.  Then we say that speciation has happened. 

We will give speciation five Lemmys because we wouldn’t even be here without it.

Advertisements

Genetics of Viking Metal, part 2 – Mitochondria

I know you are dying to find out what Viking metal could possibly be, but we have to cover something else first. I am not making this up.

My previous post explained that DNA on the male-only Y chromosome is not as mixed around from generation to generation as DNA on the other chromosomes. I also told you that DNA gets all chopped up when chromosomes are formed for egg cells.

Don’t get concerned about gender issues. Women also pass along relatively unmixed DNA exclusively to their daughters. Hold on a minute, didn’t I just say that female DNA on chromosomes gets all mixed up every generation? How can there be unmixed DNA? Simple, it’s not on chromosomes. We have two sets of DNA.

“Hold on, Rock Dad. I thought our chromosomes contained all the DNA we need.” Sort, of. This is where it gets a little sci-fi.

Let’s talk about mitochondria. Mitochondria are separate structures inside our cells that do a whole bunch of important things. Mitochondria have their own DNA. What’s freaky is that their DNA doesn’t look like human DNA. It looks like bacteria DNA. For this and other reasons, some researchers think that mitochondria originally were bacteria that became part of cells. This goes way beyond the Egyptian plover eating parasites on a crocodile’s body in exchange for “safe passage” and protection from predators. The bacterium actually became part of us and now we need each other to live. The story is really cool and would be a good topic for a future blog entry. Remind me, some day.

Sperm and egg cells both have mitochondria. The sperm’s mitochondria die soon after fertilization, so 100% of everybody’s mitochondria comes directly from their mothers. Females pass on their maternal grandmother’s mitochondrial DNA. Males don’t pass on anybody’s mitochondrial DNA.

This means that mitochondrial DNA are a link back to the mitochondrial DNA of our female ancestors. It gives us another way to track our development.

Time to wrap up. Soon I will get into how the DNA changes over many generations and what those changes tell us. We will learn about Viking Metal and I just might touch on something about hip-hop before we are through.

Sit tight and rock on.

Genetics of Viking Metal, Part 1

…or Mommy, where did Finntroll come from?

Everybody knows Vikings are cool—cooler than pirates, maybe—but I doubt that many people outside the underground metal scene know much about Viking metal. I’ll get into that soon, but first we need to talk about Vikings. We are going to use genetics to trace humankind’s prehistoric route from Africa to Scandinavia and beyond. Then we will see if we can do something similar with Viking metal. It won’t be scientific but it will be close enough for rock & roll.

Geneticists have recently started analyzing how differences between genes in different populations can show how people migrated across the planet. They have sorted hundreds of thousands of people into groups based on these differences. Then, knowing where those people live or where their recent ancestors lived, they can figure out how we have moved around.

Let’s go (back) to high school for a little while. We need to remember a few things before we can get into the cool stuff. I am going to skip the stuff we don’t need to know here. A good place to look for more info is in this Wikipedia article.

The smallest chunk of genetic material is a nucleotide. You can think of a nucleotide as a letter. The four nucleotides are abbreviated G, C, A, and T. See, even scientists think of them as letters. Nucleotides pair up. G pairs with C. A pairs with T. Nucleotides are also called bases so a pair of nucleotides is called a base pair.

String a number of base pairs together and you get a gene. A gene contains enough information to tell your body how to make a specific protein. Proteins do things for you. Each protein does a different thing.

String a bunch of genes together and you get a DNA molecule. The DNA molecule twists up into structure called a chromosome. Chromosomes also come in pairs. If you, the reader, are a human you have 46 chromosomes in 23 pair.

I would like to pretend you are female because it makes this illustration simpler. If you identify with that Who song “I’m a Boy,” you can go back to being male soon.

Each egg cell you produce will contain only 23 chromosomes—not 46. That’s exactly how you want it. If an egg cell “gets lucky” (hopefully with a sperm cell from another consenting adult), it will combine with the sperm’s 23 chromosomes to create an embryo with a total of 46 chromosomes.

So how does your body decide which 23 of your 46 chromosomes go into the egg? It wimps out and includes material from all 46. The chromosomes are in pairs, right? Each chromosome in your egg cell contains a mixture of segments of DNA from each chromosome in your corresponding pair. The mixture is different in different egg cells. If you were into hip hop a few years ago, you might call this mixing “chopped and screwed.”

Take any of your chromosome pairs as an example. One chromosome came from your mother and contains a mixture of DNA from her parents. The other chromosome came from your father and likewise, it contains a mixture of DNA from his parents. Your egg cells therefore will contain a mixture of DNA from all four of your grandparents. The same thing will be true of any sperm that happens to fertilize your egg.

The end result is that (most) DNA gets really mixed up after just a few generations. It’s too mixed up to be of use here. Luckily, there are two exceptions.

You can be male again now. Turn off your Ray Lamontagne CD and listen to some AC/DC if you need help with the transition.

When you create sperm only 22 of your 23 chromosome pairs are “chopped and screwed.” Only one of your last chromosome pair is passed on and it is passed on pretty much exactly as you got it from one of your parents. This final chromosome is the one that determines gender. Some of your sperm will have the chromosome you got from your father. Sometimes this is referred to as a Y chromosome because it looks like a Y. The rest will have the one you got from your mother. It looks like an X.

Males have one X and one Y chromosome and either one could end up in a sperm cell. Females have two X chromosomes so no matter what they will pass along an X chromosome. The DNA on the X chromosome in each egg is a mixture of the DNA from the mother’s two X chromosomes.

Since you are male, your daughters will receive an almost perfect copy of your X chromosome. Your X chromosome came from your mother. Remember that when she made her eggs, the X chromosome that would become your was a mixture of your mother’s two X chromosomes. It went through the same mixing process described above, so it’s not useful for our purpose here.

It is important to understand that your sons will inherit a Y chromosome that is almost exactly like your paternal grandfather’s Y chromosome. His Y chromosome was almost exactly like his father’s father’s father’s father’s Y chromosome. I say “almost exactly” because the DNA copying process isn’t perfect and errors are made. This is also very important and I will discuss it in more detail later.

So even though I have no idea when I last hugged my dad, we are connected by this DNA thread that leads through all our male ancestors back into prehistory. Women are better at bonding anyway, but is this one way they are ripped off? NO! They have a similar DNA link but it’s even cooler. I’ll get into that in a future post. I promise that when I explain it you will think I made it up.