Wednesday, 11 September 2013

Adam and Eve – The snake-free version that actually happened

Every so often in science you come across a fact that is at the same time strikingly obvious and obviously striking. The most memorable example I can think of in my own life was the day that I learned that Adam and Eve were real people.

Before we let our imaginations get the best of us, let’s stop and recognize that I am not talking about the biblical Adam and Eve. It doesn’t take a brilliant mind to see that two people with three sons would have a hard time spawning the rest of humanity without some serious and deleterious in-breeding going on. No, I am talking about the two people known to science as Y-chromosomal Adam and Mitochondrial Eve. Although it is all but certain that they never crossed paths, these two people are literally the direct ancestors of every person alive today.

In a way, it is common sense. Since every living thing on Earth shares a common ancestor (if you go back far enough in time) there must be a point at which all human bloodlines converge. Yet, this is one of those cases where common sense can still knock you off your feet.

To really understand what I’m talking about, you need to understand something about genetics (don’t worry, it’s a simple idea). Basically when a man and a woman do the horizontal hustle and produce a baby, it is largely a product of both of their DNA. Genetic material from the man mixes from that of the woman and gets recombined to make a new genetic code that is similar to but different from the genetic code of each parent. That is how most genetic change happens, but it isn’t the only way.

The alternative is mutation. As we have seen in previous articles (mitosis, epigenetics, and radioactivity) mutation can happen as a result of things we are exposed to (radiation, diet, etc.) but it can also happen naturally. Natural mutation happens at a fairly steady rate that has been estimated at 0.00000003 mutations per generation.

Y-chromosomal Adam, as the name suggests, is the man from whom all living men have inherited their Y-chromosomes. Since only men carry Y-chromosomes, they are only passed on by fathers, grandfathers, great-grandfathers etc. and they are not influenced by DNA from mothers. As a result, the only way the Y chromosome ever changes is through mutation.

This is not just a logical idea; it is something scientists have demonstrated. By looking at the Y-chromosomes of men from all over the world and counting the number of mutations, researchers have estimated that “Adam” probably lived sometime around 100,000 years ago in Africa. He was probably more concerned with outrunning hyenas than fathering 3.5 billion sons, but somehow he managed to pull it off.

We can use a similar idea to determine how long ago the mother of all humans lived, too. Instead of using Y-chromosomes, scientists have looked at mitochondrial DNA. Mitochondrial DNA is a separate set of DNA that doesn’t really belong to you but is found in every one of your cells. You see, every cell contains organisms called mitochondria which convert food into energy. The weird thing is, billions of years ago, mitochondria were totally separate and independent from cells. At some point, they made a deal with the organisms that would become cells where they would live inside them and provide energy to their hosts. They never fully integrated though, and to this day mitochondria still replicate on their own schedule and have their own DNA.

Sperm don’t have mitochondria, but eggs do. The upshot is that, whether you are a man or a woman, you got all your mitochondria from your mom. Similar to Y-chromosomes, this DNA only changes through mutation and, as a result, we can trace it back to the point where all human bloodlines converge. It turns out Mitochondrial Eve lived about 200,000 years ago, also in Africa.

These were the numbers that were generally accepted until a couple weeks ago. It turns out that two teams of researchers working independently of one another have revised the age estimates of the mother and father of humanity. Their work suggests that Adam lived between 120,000 and 200,000 years ago and Eve lived sometime between 99,000 and 200,000 years ago.

Look at those numbers again. It is pretty amazing. The ranges now overlap. That means that the two people at the root of all human DNA might have even been alive at the same time on the same continent. Admittedly, it isn’t very likely but the possibility is amazingly cool. At some point there may have been two people who never knew each other and never spoke to one another and probably were never within a few hundred miles of one another; but through a brilliant streak of genetic luck, they produced every single one of us. Chew on that for a while, cousin.

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