Friday, February 28, 2014

10,000 years living on a bridge

This simulated map shows the theorized extent of the Bering
Land Bridge between Russia (left) and Alaska (right), by
which the first Americans are thought to have migrated
from Asia.  Credit: William Manley, University of Colorado.
Image source.
It is generally accepted in the scientific community that most of the original Native Americans arrived in North America from Asia via a bridge of land (often referred to as "Beringia") that appeared between Russia and Alaska during the Ice Age due to lower sea levels.  Native Americans are most genetically similar to certain Asian groups, so it is reasonable to believe that Asia was their place of origin.  Generally, creationists accept this migration theory, though they of course claim it happened more quickly and more recently.

A recent publication in Science magazine, however, would initially appear to contradict the creationist timeline.  The article itself is not a scientific study, but it combines the findings of several studies and places them within a broader context.  According to the authors, the migrating humans lived in Beringia for 10,000 years before spreading throughout North America.  This amount of time is greater than creationists say the earth has even been around, so how does a young-earth creationist account for such findings?

First, it should be made clear that there was no direct evidence for the 10,000 year stay in Beringia.  Rather, the authors reference genetic studies that have dated the separation of Asians and Americans to 25,000 years ago, and they compare these with studies suggesting that Native Americans did not spread throughout North America until 15,000 years ago.  This leaves a 10,000 year gap for which there apparently is no archaeological evidence of any North American inhabitants.  To solve this discrepancy, the authors proposed that those 10,000 years were spent in Beringia, which is now underwater and has not been searched for archaeological remains.  They additionally cite paleoecological studies that suggest that Beringia was quite livable during that time.

What about the dates for genetic separation and for American dispersal?  Those dates were calculated from mitochondrial DNA (mDNA) mutations in modern Native Americans.  Unlike nuclear DNA, mDNA resides in the mitochondria of the cell, where most of the energy is produced.  It is only inherited from the mother and seems particularly prone to mutations, so it is frequently used in studies of familial and evolutionary relationships.  As with radioactivity, a rate can be ascribed to certain mutations based on their likelihood of occurrence.  In theory, these rates would be precisely measured in a laboratory, but they do not occur frequently enough to get accurate measurements.  Instead, the clock is "calibrated" using evolutionary assumptions.  Because scientists "know" that humans split from chimpanzees 5 to 7 million years ago, they can divide the differences between human and chimpanzee mDNA by the time since their last common ancestor, thus arriving at a rate of mutation.  This rate was applied to the differences between Native American and Asian mDNA to arrive at the dates cited in the paper.  Of course, if humans didn't share a common ancestor with chimpanzees 6 million years ago, then the rate would be thrown off and likely highly underestimated, resulting in ages vastly older than reality.

We can conclude, then, that the Native Americans likely did originate from Asia, but studies such as this cannot be used to refute the creationist model of origins, as they require old ages and evolutionary relationships to begin with.  This is not to say that assumptions are wrong; both creationists and evolutionists need to make assumptions.  However, we must be keenly aware of what those assumptions are when determining how the evidence impacts the origins debate.

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