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Home arrow Books arrow The Journey of Man -- Spencer Wells

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The Journey of Man -- Spencer Wells Print
Written by Mike Noel   
Sunday, 18 May 2008

Journey of Man

In The Journey of Man author Spencer Wells puts forth a description and explanation of how modern man descended from a single man located somewhere in Africa around 60,000 years ago.  In more detail Wells describes the actual migration routes over the millenniums showing how man moved from one continent to another and eventually spread to every corner of the globe.  Wells is able to come up with these astounding details through the clever use of genetics, in particular the Y chromosome, and statistical analysis. 

Unfortunately there are many many details left out and lots of gaps that make the whole premise much less solid.  As I read through the book I found myself frequently scratching my head and wondering how the conclusion could possibly follow from the facts presented.  There was also very little description or explanation of how Wells obtained the genetic data that he bases the entire premise on.  

First of all let me say that I understand that this book was not meant to be written at a technical level.  It was intended to be accessible to the average Joe who is not a student of genetics, biology, or evolution.  Necessarily this means that certain details must be abstracted out, passed over, or just given as fact.  However, I feel that too much of this went on.  Enough that it's not possible to critically evaluate what the author is proposing.  But still, I'm willing to consider that the problems I had with the book are resolved in other more details works.

One of the first problems I encountered was the issue of genetic drift.  This was a new concept for me so I was trying to figure out what it meant.  Wells seemed to have described it as something akin to statistical sampling error.  That is, I know that a coin toss will end up 50% head and 50% tails.  Yet, if I toss a coin 10 times I may get 7 heads and 3 tails.  Does that mean that the real expected result is 70% and 30%? No, it's just a sampling error.  In the same way, if I see a population that has 70% brown eyes and 30% blue eyes that doesn't mean that the actually ratio is 7:3.  It could be sampling error.

I asked a co-worker about this definition of genetic drift and he said it was wrong.  Instead, he defined genetic drift as the natural attenuation of certain genetic traits in a population simply due to the fact that with each generation there are fewer individuals in the population with that trait and therefore fewer chances for those individuals to create new offspring with that trait.  Over time the trait simply disappears because there aren't enough individuals propagating it.

Wells claimed several times that the rate of genetic drift was a known quantity.  He used this to be able to map genetic changes back in time.  However, I don't see how genetic drift can be measured (using either of the definitions supplied above).  So I'm guessing that my understand of what genetic drift is in wrong.

Another major assumption that Wells makes is that if two different gene sequences show the same aberration then they must have descended from the same source.  In the general case this isn't true.  A table has four legs and a cat has four legs, that doesn't mean that they both have a common ancestor.  There has to be more involved for us to believe that there is a common ancestor ("common sense" doesn't count as an explanation) and I think that Wells tries to provide that.  He alludes to the statistical probability of the same aberration appearing on independent ancestral chains as being so small that it only makes sense to assume that they are the same.  Without actually seeing the numbers that makes sense.  At one point in the book he reports that about 1 out of 1000 gene replications results in an error.  With millions of base pairs making up the gene sequence it is not unusual to see thousands of replication errors (mutations) in each generation.  So then, if we are having mutations at that rate, then maybe it isn't so far fetched to think that the same mutation would happen on completely different evolutionary lines.  This concept of an "evolutionary clock" is not new to Wells and has been studied for the last few decades.  A little bit of google research shows that there are some misgivings (amongst scientists who know) about the veracity of this technique.

Both the genetic drift and the "evolutionary clock" are crucial factors that Wells relies on in order to help establish his time line.  Without these two items he would not be able to create a time line for how man evolved.

Another issue I had, and the last one I'll mention in this review, dealt with the source of his genetic material.  This is the most perplexing of the issues for me. Simply put, where is he getting the organic material for extracting DNA?  You can't get DNA out of fossils since the organic matter has been replaced with rock (in a manner of speaking).  Wells makes many claims correlating the genetic age, as determined using genetic mutations, with geographic regions.  By doing this over the entire 50,000 year time period he is able to plot a map showing the migration pattern over this same time period.  In order for this to work he has to show that a certain DNA sequence was at a certain location at a certain time.  I'm not sure how he does that unless he has DNA samples that are of a corresponding age. Some simple google research shows that there are DNA samples reaching back as far as 60,000 years so it is possible that that's where he got his data.  I would have liked to see that explained a bit more.  

Before anyone starts yelling at me about how ignorant I am of everything related to this book let me say that I know that.  I took one biology class in high school and then one in college. That was the extent of my exposure to these issues.  However, I was led to believe that this book was written for the layman and that it would put to rest the whole idea of human origins.  Unfortunately it raised more questions than it answered for me.  I'm used to analytical thinking but I felt there were lots and lots of holes in the presentation.

Despite all of that this has piqued my interest in this topic.  I will likely pursue it more and learn more about it.




Last Updated ( Friday, 30 May 2008 )

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