Book Summary: The Book of Humans: A Brief History of Culture, Sex, War and the Evolution of Us
Author: Adam Rutherford
Species are defined by their morphology and not by their DNA. That taxonomy exists for historical reasons: we were classifying organisms using the current system since Linnaeus devised his binomial nomenclature in the eighteenth century – genus followed by species, Homo and sapiens, Pan and troglodytes. Every human being has a unique genome, but they are similar enough that we can be sure that we are one species. Crucially, all living humans typically have the same number of chromosomes. Each chromosome is a long thread made of DNA, and parts of each thread are genes, around 20,000 of them for us, spread over those twenty-three pairs of chromosomes. Gorillas, chimpanzees, bonobos and orangutans have twenty-four.
Chromosomes are all different sizes, and our number 2 is one of the biggest, representing about 8 per cent of our DNA, and harbouring around 1,200 genes. It’s that big because at some point, maybe six or seven million years ago, one member of the common ancestors of all the great apes gave birth to a child with a gross chromosomal abnormality. During the formation of the egg and sperm that would fuse to begin this life, instead of replicating all the chromosomes perfectly, somehow two of them crunched together and stuck. By lining up all the great apes’ chromosomes, we can see very clearly that the genes on our chromosome 2 are spread over two different chromosomes in chimps, orangutans, bonobos and gorillas.
Most mutations of this magnitude are utterly lethal, or cause terrible diseases, but this ape got lucky, and was born with a fully functional genome that was significantly different from his or her parents. From that point on, the genealogical lineage of twenty-three pairs of chromosomes would trace a line all the way to you.
We now have the full genomes of other types of humans, the Neanderthals and the Denisovans, but annoyingly, chromosome count is not preserved in the fragmented DNA that we can get from their bones. We reasonably suppose that they also had twenty-three pairs due to their relatedness to us, but we cannot be absolutely sure, until we get much better quality samples out of the sparse DNA-laden bones. We know we bred with them, and a different number of chromosomes is often a very sturdy barrier to reproductive success, though not always: living equids – that is, species of horse, ass and zebra – show clear evidence of having interbred despite having chromosomes varying between sixteen and thirty-one pairs. No one has figured out how though, yet.
We haven’t been able to extract DNA from most specimens from the ancient human family tree, and may never be able to, as so much of our ancestors’ remains are from Africa, where heat renders preservation of DNA fairly untenable. It is likely that all apes after the split from what would become chimps, bonobos, gorillas and orangutans, have twenty-three pairs of chromosomes.
Genes are translated into proteins, and proteins perform actions in bodies. This includes everything from forming hair or the fibres in muscle cells, to manufacturing the components of cells that are fatty or bony, or acting as the enzymes and catalysts that process food or energy or waste. Subtle variations in genes result in changes in the shape or efficiency of proteins, and that means that some people have blue eyes and some have brown,2 or that some people can process milk after weaning, but most can’t, or that some people’s urine smells after they’ve eaten asparagus and other people’s doesn’t (and some people can smell it and others can’t). Genetic variation becomes physical variation. We call the specific sequence of DNA the genotype, and the physical characteristic it encodes the phenotype.
DNA changes randomly, and these mutations are subject to selection if the phenotype is beneficial to the survival of the organism, or impairs it. Over time, bad mutations are generally weeded out, because they reduce the overall fitness of the creature that bears them, and good ones spread. Sometimes it’s a bit of both: having one defective version of the beta-globin gene acts as protection from malaria; having two copies means you get sickle cell disease. Many simply drift – the genetic mutations encode change that is neither good nor bad.
Though we have almost the same set of genes as the other great apes, many of those genes are slightly different, and a few of them are new to the human genome. Those differences are us. There are lots of ways that, over generational time, genes and genomes can change and create new information. They can subsequently be selected, in a direction that may ultimately become a unique combination for a distinct species. I won’t go through all of them, as all happen in all creatures. But some mechanisms by which mutation occurs are pertinent to the formation of our uniquely human genome and are worth looking at more closely.