The Original People
Once upon a time, paleontologists believed that human beings and the other primates went their separate ways 15 to 30 million years ago. They based the conclusion on inexact and wide-ranging dating of bits of teeth and skulls scattered throughout the world. Moreover, paleontologists found bits and pieces of big-brained, upright-walking hominids they believed gave rise to modern humans in sites throughout Asia and Europe. Scientists in the early 20th century therefore concluded that the ancestors of modern humans had spread across much of Eurasia and Africa millions of years ago. These distinct, local populations might then have mingled to form the stock for modern humans.
But some 30 years ago, scientists with new research and new ideas challenged the conclusions of the fossil hunters. The new work was based on a careful analysis of the genetic blueprint coded into the spiral of DNA. Every cell contains a coil of DNA in its nucleus. This coil of amino acids contains the instructions for making all of the complex proteins that comprise the cell and enable it to carry out its business. Initially, geneticists thought human beings had about 100,000 genes, but recent work has reduced the number of actual genes to about 30,000. Living species change and evolve largely because of changes in their DNA. Any genetically based characteristic that helps a person, bacteria, or elephant leave more children will tend to spread through the population.
But while the 30,000 functioning human genes are therefore subjected to this continual pruning and selective pressure, many stretches along that DNA spiral are just accumulated random junk, no longer functioning as genes and therefore no longer pruned and discarded by the process of genetic evolution. These noncoding pieces of DNA therefore accumulate without changing as much as the actively evolving genes. Geneticists sought to take advantage of that fact by using these randomly accumulated genetic bits to compare one species to another. Moreover, elaborate experiments revealed that these random, noncoding bits accumulate at a fairly constant rate over the course of hundreds of thousands of years, which means they can be used as a molecular clock.
This discovery opened the door to a scientific revolution. Geneticists discovered they could directly compare the DNA of any group of creatures, compare the accumulation of these random bits, and get a rough idea as to the last time those two species had identical DNA, which would indicate when they last shared a common ancestor.
These scientific innovators compared the DNA of human beings to their nearest relatives, chimpanzees and gorillas. To their surprise, the DNA-based molecular clock suggested that human beings, chimpanzees, and gorillas all shared a common ancestor as recently as 5 to 7 million years ago. In fact, humans, chimpanzees, and gorillas share about 98 to 99 percent of the same genes.
Another equally startling discovery soon followed. Human beings are almost genetically identical to one another, at least as compared to other species. Despite the apparently significant differences in appearance between different human populations and races, the genetic differences between various groups are tiny. Two different subspecies of seemingly identical sparrows actually have much greater genetic differences than, say, Native Americans and Europeans. That finding suggests that all current human populations shared a common ancestor in the recent past.
So researchers gathered DNA samples from virtually every human population in the world. They then set out to compare all these coils of DNA. Seeking a way to make the analysis easier, they focused on a curious little cell-within-a-cell, the mitochondria. Modern cells probably represent an alliance long ago between two microscopic organisms, the ancestor of the cell and the ancestor of the mitochondria. Now, every cell uses this strange structure to essentially produce the energy it needs.
Oddly enough, the mitochondria floating around inside the much larger cell has its own microscopic coil of DNA, a sign of its once-independent existence. But the mitochondrial DNA comes only from the mother's egg, which means it does not split and take half of its DNA from the sperm cell of the father like the main coil of DNA in the cell. Therefore, the DNA in the mitochondria gets shuffled and reshuffled much less often than the DNA in the main part of the cell. That makes the mitochondrial DNA perfect for charting the random accumulation of mutations that makes the molecular clock analysis possible.
The comparison of human mitochondrial DNA yielded some remarkable findings. First, all human beings living today are descended from the same woman who may have lived as recently as 70,000 years ago, although some studies put the date at 120,000 to 180,000 years ago. Moreover, if you charted the variation between all the different human groups, the Bushmen of the Kalahari sit right in the middle, indicating that all other groups are equally related to the Bushmen. This suggests that the DNA of the Bushmen is closest to the root of the human family tree. That conclusion also matches the fossil evidence. Therefore, most experts now believe that modern human beings evolved on the semiarid desert plains of Africa and spread throughout the world from there.