To understand what genes tell us about human beings, it would be wise to start with what genes can tell us about the origins of human beings.
We can use genetic diversity as a way to measure the age of any species. Mutations – changes in the structure of genes – accumulate over generations. In other words, your brother or sister has more in common with you genetically than your grandad or grandma. Genetic diversity acts as a timekeeper and the time difference – i.e. intergenerational time – between any two family members is proportional to the genetic diversity between them.
Where we look for the mutations is crucial to our investigation. In most human cells, genes can “cross over” between paired chromosomes – thread-like structures which store genetic information – generating variation without mutation. Fortunately, there is an isolated corner of the genome where change can occur only through the accumulation of mutations, thereby giving us the perfect “molecular clock”.
Every cell possesses a subcellular structure called a mitochondrion that is used to generate energy. Mitochondria have their own mini-genome, with only thirty-seven genes. Strangely, mitochondrial genes resemble bacterial genes more than human ones. Some scientists believe that mitochondria originated from some ancient bacteria that invaded single cell organisms.
In almost an identical manner to how Venom forms an alliance with Eddie Brock, these bacteria formed a symbiotic alliance with the host; they provided energy, but used the organisms cellular environment for nutrition, metabolism and self defence. The genes within mitochondria are left over from this ancient symbiotic relationship. Mutations in mitochondrial genes are passed intact across generations, and they accumulate over time without crossing over, making the mitochondrial genome an ideal genetic timekeeper.
So what did we find out about our past from the mitochondrial genome? First, we are not so different from each other. You may look at chimpanzees and think that they all look the same but in fact, they are probably looking at us and thinking the exact same thing and they are closer to the truth than we are. The difference between human genomes is less than that of chimpanzees. We are only two hundred thousand years old after all, which on the scale of evolution is negligible. In contrast, dinosaurs existed for approximately 179 million years.
Second, we all came from the same place, somewhere in sub-Saharan Africa and some of our ancestors then migrated northward and eastward to populate the Middle East, Europe, Asia, and the Americas. There is less genetic variation the further away you go from Africa. Each small group of people that broke away to find new land, took only a sample of the parent population’s genetic diversity.
Lastly and most significant of all, all humans have inherited their mitochondria from their mothers. This means we can trace our mitochondrial lineage to a single human female who existed in Africa about two hundred thousand years ago. She is the common mother of our species. While we do not know what she looked like, her closest modern relatives are the women of the San tribe from Botswana or Namibia. To put it plainly, we are all just part of one big family.
Inspiration: Siddhartha Mukherjee’s The Gene: An intimate history
Image credit: Shinyoung Kim