Okay, everyone, buckle your seatbelts and put on your thinking caps. In this week's installment in my continuing series, we'll be examining two (count 'em!) pieces of the evidence and how they relate to each other.
You can check out my previous articles here:
Retroviral sequences: http://www.jehovahs-witness.com/6/86797/1.ashx
Cytochrome c: http://www.jehovahs-witness.com/6/87238/1.ashx
What evolution is not: The role of randomness: http://www.jehovahs-witness.com/6/87711/1.ashx
First, a disclaimer: I have been studying this over the past week in order to write this article, and I think I understand the science fairly well. But I would appreciate it if the other evolutionarily inclined readers make contributions/corrections where necessary.
Primary sources for this article:
Hominids and mtDNA: http://www.talkorigins.org/faqs/homs/mtDNA.html
What is Mitochrondial Eve?: http://www.talkorigins.org/faqs/homs/mitoeve.html
Paleoanthropological Discovery Timeline: http://www.talkorigins.org/faqs/homs/recent.html
Overview of Neandertals: http://www.answers.com/topic/neandertal
Refutation of Common Creationist Arguments Regarding Neandertals: http://www.talkorigins.org/faqs/homs/a_neands.html
Mitochondrial DNA
To start out, we should briefly review DNA. DNA is composed of two extremely long sequences of chemicals called base pairs which are like rungs on a ladder that has been sawed in half from top top bottom. The two sides of the ladder attach to each other (forming a complete ladder), and twist into the familiar double-helix shape.
Each of our cells contains the complete set of DNA for building and repairing everything in our bodies. Each cell contains two copies of the complete DNA sequence, inside the nucleus of the cell. Therefore, this DNA is called nuclear DNA. However, it turns out that nuclear DNA is not the only DNA we have.
Inside each cell are energy-producing organelles called mitochondria. Surprisingly, mitochondria carries its own miniature set of DNA. Mitochondrial DNA, or mtDNA for short, is much shorter than nuclear DNA, containing only about 16,000 base pairs, as opposed to about 4.8 billion in nuclear DNA (reference: http://www.pbs.org/wnet/secrets/case_amazon/clues.html). This makes sense, of course, because nuclear DNA is coding for the entire body, whereas mtDNA codes only for mitochondria-related proteins and acids.
Here's where it gets interesting. Whereas our nuclear DNA is passed on to us from both our fathers and our mothers, and is thus always different from either of our parents, mtDNA is passed only from the mother, because it comes from mitochondria in her ovum. (There are rare exceptions to this; sperm cells contain mitochondria in their tail, but it is used to power the sperm to the ovum and is depleted by the end of its journey. Additionally, the tail enters the ovum only very rarely anyway, so transmission of male mtDNA is an exceedingly rare event.)
There are a couple consequences of these facts. First, mtDNA is usually exactly the same in people closely related through matrilineal descent. For example, you and your siblings probably have identical mtDNA. However, a cousin on your dad's side would likely have slightly different mtDNA, because you and her do not share a close common ancestor through matrilineal descent (although you do share a grandfather in common that can be reached by father-to-father descent).
Because mtDNA does not code for our personal genetic traits, but only for mitochondria-related machinery, it is the same in closely related people. For example, when a key section of the mtDNA of Europeans is compared, only 1% of the population differ by 12 or more base pairs. The same goes for Asians. Africans have slighly greater variance, with 37% differing by 12 or more base pairs.
Another fact about mtDNA is that because it is in every one of the 500-1000 mitochondria you have in each cell, and it is much shorter than nuclear DNA, it is much easier to retrieve and work with.
Because mitochondria only changes when a chance mutation causes it to, and we have been able to observe how often such mutations occur, by comparing the mitochondrial DNA of humans all over the world, we can get an idea of when they diverged from their most recent common matrilineal ancestor.
To understand this, let's imagine a woman has several daughters. She passes on mitochondrial DNA to them. Each of those daughters has children, passing on the same mitochondrial DNA to them. Now when we look at the mtDNA of those grandchildren, we notice that they are identical. Therefore, we know that their most recent common matrilineal ancestor was very recent indeed, only two generations ago, as it turns out.
If we find two people with almost identical mtDNA, it means that they have a common matrilineal ancestor sometime in the near past, but enough time has gone by to allow slight mutations to occur in the different lines, which gradually built up as they were passed down from mother to mother. The greater the differences between the mtDNA in any two humans, the more time has elapsed since their most recent common matrilineal ancestor lived.
By comparing the mtDNA of humans all over the world, we find that our most recent common female ancestor, by matrilineal descent, lived about 150,000 - 200,000 years ago. It is vitally important to understand that this does not support the concept of a Biblical Eve character - quite the opposite, actually. Please see the following references and be prepared to think if you'd like to learn about why:
http://www.freemaninstitute.com/RTGdna.htm
http://www.talkorigins.org/faqs/homs/mitoeve.html
Whew! So that's the introduction to mitochondrial DNA. I'm going to have to put off the discussion of Neandertals and how mtDNA relates to them until my next installment, because I'm plum out of time for this week.
SNG