There is science behind the science we read about in textbooks and headlines and even more science behind that.
A telling case: there is general agreement among scientists who study human evolution, that modern humans appeared about 200,000 years ago in Africa.
However, there has not been agreement about how these humans left Africa and spread across the Earth (multiple migrations? single migration? 100,000 years ago?).
Finding an answer to these questions has been a major preoccupation of those who study human evolution. (Zimmer, Matter, 9/21/2016)
In September of this year, three articles and a commentary appeared in the journal Nature proposing an answer to these two important questions about human evolution: when did humans migrate out of Africa and was it done in stages or all at once?
While there is a large body of genomic data about human evolution (most of this has been collected from people living in major population centers in North America, Europe, and Asia), the researchers recognized that in order to understand the migration and geographic spreading of the early human population it is necessary to examine the DNA of indigenous populations "such as Basques, African pygmies, Mayans, Bedouins, Sherpas, and Cree Indians" to ensure that their analyses would be representative of the diversity of the world's population. (Zimmer, Matter, 09/21/2016)
After the collection, sequencing, and analyzing of these genomes, the three research teams independently concluded that while "we know there were multiple dispersals out of Africa...we can trace our ancestry back to a single one," that occurred between 80,000 and 50,000 years ago. (Zimmer, 2016)
It was those migrants who became our ancestors whatever our geographic homeland or our ethnic background.
Such conclusions are based on other science.
While it is obvious how DNA analysis can show relationships between people in the same families or can even be used to connect evidence at a crime scene with specific individuals, how can DNA be used to tell time, for example, that modern humans appeared 200,000 years ago? The science of the gene is right at the center of biology.
In order for a fertilized egg to become a fruit fly or a human being it needs information about how to do this. The gene provides the means by which information is passed onto the future from one generation to the next.
The gene looks backwards as well in that it contains a record of all of the mutations that have occurred to it, to parents, grandparents, great-grandparents, and great-great-great-as far back as possible. It is this complete genealogy that is passed along to the next generation.
It is this fact that provides the mechanism by which the gene tells time.
In the early 1960s Emile Zuckerkandl and his colleague Linus Pauling observed in their studies of the hemoglobin molecule that the rate of mutations to the molecule was relatively constant over time, and they proposed that as generations pass, the accumulation of mutations is like the ticking of a clock.
The genome records the changes that result from mutations, with each mutation being added to the information carried by the gene as well as marking the passage of time.
Their insight was that a genome was not only the blueprint for building a human being, it also embodied a database of the evolutionary history of its human.
"Living humans are endowed with the evolutionary history of our species in our genomes. It is as if we permanently carry a photograph of each of our ancestors in our wallets." (Mukherjee, 2010, p. 335)
The technology that made it possible to actually use the genome as a clock was developed by Allan Wilson, a Berkeley biologist in the 1970s using tools not available to Pauling-Zuckerkandl along with the development of new ones.
Wilson saw that the molecular clock could be applied not just to individual families "but across an entire population of organisms. Variations in genes could be used to create a mmap of relatedness." (Mukherjee, 2016, p. 334)
Wilson also realized that the molecular clock was made less accurate because of the fact that when we inherit genetic material from each parent, genes from the mother-contributed chromosome can occasionally cross-over to the father-contributed chromosome. The cross-over is the reason behind the appearance of the dark-haired, blue-eyed or the dark-eyed blond child. This adds an unknown to the mutation record.
To get around this problem, Wilson used genomes found in human cellular mitochondria. We inherit this genome from our mothers only so there is only a single copy so mutations are passed along faithfully without dangers of cross-overs.
Using the mitochondrial genetic data from large databases, Wilson and his students made a number of important discoveries.
When Wilson used mitochondrial methods to compare human mitochondrial mutations with those of the chimpanzees, our closest non-human relative, he and his collaborators found that the human lineage separated from that of the great apes about five million years ago. They also found that by counting backwards through the generations that modern humans appeared about 200,000 years ago.
After Wilson's death in 1991, his students as well as other scientists expanded its scope and added new techniques and vast amounts of data from humans across the globe.
Investigations identified the oldest human genomes were found among the KhoiSan people of southern Africa, hence humanity's African genesis. (Mukherjee, 2016, pp. 335-6)
The research done in 2016 is not simply a remarkable use of genomic tools to propose an answer to the question of human migrations, it also demonstrates that the society of STEM is not only based on scientists and their contemporaries but also on the work of scientists, engineers, mathematicians, and technological innovators from both the near and not so near past.
An insight based on an observation from studies of evolutionary changes in an amino acid made half a century ago and then developed by another investigator who made the "molecular clock" less prone to error by using mitochondrial DNA made it possible for other scientists to develop a solution to the problem of how the Earth was populated.
Malaspinas, A.-S. et al. Nature http://dx.doi.org/10.1038/nature18299 (2016).
Mallick, S. et al. Nature http://dx.doi.org/10.1038/nature18964 (2016).
Mukherjee, Siddhartha The Gene: An Intimate History.
Pagani, L. et al. Nature http://dx.doi.org/10.1038/nature19792 (2016).
Tucci, Serena & Joshua W. Akey (2016). A map of human wanderlust. in Nature News & Views.
Zimmer, Carl. “A Single Migration From Africa Populated the World, Studies Find. New York Times, September 22, 2016 on Page A1 “How We Got Here: DNA Points to a Single Migration From Africa.”
Dr. John Holton
Dr. John Holton joined the S²TEM Centers SC in July of 2013, as a research associate with an emphasis on the STEM literature including state and local STEM plans from around the nation.