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Out of Africa: retracing human evolution and migration with DNA
The fossil record is compelling evidence that points to the dawn of man in eastern Africa. Now that we have advanced DNA sequencing capabilities, what else can technology tell us about evolution and human migration?
In July of 1959, a remarkable discovery was made in the Olduvai Gorge of northern Tanzania, along the fertile Great Rift Valley of East Africa. Drs. Louis and Mary Leakey (the husband and wife paleoanthropology team) were excavating this area because earlier work by geologist Hans Reck indicated possible hominid fossils. The Olduvai Gorge is the site of an ancient lake that once teemed with life followed by an era of erosion that exposed fossils in a “layer cake of evolution”.
That fateful morning Louis Leakey fell ill and stayed at camp. Mary Leakey spotted an unusual rock jutting out from the ground which proved to be a fossilized jaw bone with teeth. Excavation revealed the cranium of what she named “Nutcracker Man”, a 1.75 million year old ancestor of the first humans. This was one of many discoveries that support the theory of humans originating from Africa. It is also an example of a major contribution by a female scientist. Dr. Mary Leakey continued her husband’s work when he died in 1972.
Science and technology have advanced by leaps and bounds since 1959, so what can current 21st century science tell us about human evolution? More specifically, what is the DNA evidence? Unfortunately, time and desiccation makes it impossible to extract DNA from hominid fossils that are millions of years old. Also, the human genome recombines with every generation, which is useful in paternity cases but proves to be more difficult when studying evolution over the eons. What is needed are genetic markers in living humans that change very slowly over time.
The sex of a child is determined by the father. The mother donates one of two X chromosomes, whereas the father donates either an X female chromosome or a Y male chromosome. A daughter is born XX and a son is born XY. The other 22 pairs of human chromosomes recombine each generation, but the Y chromosome passes from father to son relatively intact. Therefore, father-son lineage can be tracked over the ages to the first male human which has been called the chromosomal Adam.
Mitochondria inhabit the cytoplasm of cells outside of the cell nucleus (where the 23 chromosome pairs are located). Mitochondria are essential in cell energy production and have their own distinct DNA. Because the human ovum cytoplasm is about 60,000 times larger than the sperm, we inherit our mitochondria from our mothers. Therefore maternal lineage can be tracked over the ages to the very first human female which has been called the mitochondrial Eve.
With modern DNA extraction and sequencing techniques, it is possible to study the DNA in living human populations to trace human evolution and migration all over the globe by analyzing these Adam and Eve genes. For example, the DNA evidence confirms the African origin of humans and proves the land bridge theory of human migration from Asia to the Americas. Every human race and nationality has relatives from Africa. Although national and cultural pride emphasizes diversity, our biology proves our commonality.
In my recent book with legendary cyclist Greg LeMond, we explain fitness as a biologic process powered by mitochondria. As I researched the book, I discovered why LeMond’s athletic genes were so good. VO2 max is the best laboratory test of aerobic fitness and is a measurement of an athlete’s mitochondrial “engine”. LeMond’s VO2 max of 92.5 ml/kg/min was one of the highest ever recorded in an athlete. With that engine, LeMond had the speed to win time trials, the power to break away on mountain climbs, and the endurance to win three Tours de France. What made his mitochondria so strong?
Although Greg is blond-haired and blued eyed, his mother’s mother was Cherokee and when his DNA was tested it turned out that he is 18% Native American. Greg’s mitochondria descend from an ancient lineage of hardy well-traveled people who walked half way around the world on foot. Greg’s first training partner was his father, who was an avid cyclist, so some athletic ability came from his father. It should also be noted that Greg’s innovative high intensity training regimen is known to trigger mitochondrial biogenesis (multiplication). Genetics are a gift that determine what is possible, but physical training is what allows athletes to reach our full potential. If LeMond hadn’t started competing as a teenager, he never would have realised his athletic ability.
The Family Tree shows that all life on Earth evolved from a common origin and is connected by mitochondrial DNA ancestry. Image by Mark Hom.
Besides human origins and athletic power, there is even more that we can learn from mitochondrial genetics. Mitochondrial DNA goes back to the very first plants and animals and as far back as single-cell creatures. When mitochondrial DNA is analyzed over span of 2 billion years, we can see the human branch in the grand evolutionary tree and how we are intimately related to every other living thing.
