Ridley states that in 1955 it was agreed that human beings had twenty-four pairs of chromosomes. This was considered a fact because in 1921 a scientist named Theophilus Painter had sliced thin sections off the testicles of three castrated men and examined them under a microscope, arriving at the figure of twenty-four. This number was accepted as true for thirty years, even though some scientists were unable to find that number of pairs in human liver cells. It was not until 1955, when two scientists, Joe-Hin Tjio and Albert Levan, using better techniques, plainly saw that there were only twenty-three pairs of chromosomes in human beings. They even went back and counted twenty-three pairs in photographs in books where the caption stated that there were twenty-four pairs.
Ridley states that it is surprising that human beings do not have twenty-four pairs of chromosomes like chimpanzees, gorillas, and orangutans. The difference between humans and other great apes is that humans have one pair less of chromosomes. The reason for this is that two ape chromosomes have fused together in humans. The Pope John-Paul II argued that this fusion was the point at which God injected a human soul into an animal lineage, but Ridley argues that this is not the case. He states that natural selection is simply the process by which life-forms change to suit the opportunities in the environment, and that there is no such thing as evolutionary progress. He also points out that some other species, such as the black-smoker bacterium, living in a sulphurous vent on the floor of the Atlantic ocean, are arguably more highly evolved than human beings, at least at the genetic level, because it has had more time to perfect its genes
Ridley states that this book’s focus on the human species does not indicate its importance relative to other species. He acknowledges that human beings are unique and have the most complicated biological machine on the planet, but argues that complexity is not the goal of evolution. He also notes that every species is unique and that uniqueness is in oversupply. He then goes on to explore the unique characteristics of the human species and argues that human beings are an ecological success. They are one of the most abundant large animals on the planet, rivaled only by those that have been domesticated or are dependent on man-made habitats. Humans have also shown a remarkable capacity for colonizing different habitats. He also notes that this ecological success comes at a high price and that humans are doomed to catastrophe soon.
Ridley states that human beings are a successful species but also points out that they are a result of a long line of failures. He explains that the human species is a part of a group of apes that almost went extinct 15 million years ago in competition with the better-designed monkeys. He also mentions that the human species is a part of a group of primates that almost went extinct 45 million years ago in competition with the better-designed rodents. He states that the human species has an ecological success that came against humbling odds and that their ecological success is a result of the long process of trial and error, known as natural selection. He explains that about a billion years ago, life on earth saw a new world order, with the invention of bigger, multicellular bodies and that this sudden explosion of large creatures led to the growth of large and complex creatures such as trilobites and worms. He concludes by stating that the journey from four billion years ago brings us to just ten million years ago, past the first insects, fishes, dinosaurs, and birds, to the time when the biggest-brained creature on the planet was probably an ape, which gave rise to human beings and chimpanzees.
Ridley states that calibrating the molecular clock to give an actual date in years is difficult. The generally accepted range for the split between chimpanzees and human beings is five to ten million years. He also states that apart from the fusion of chromosome 2, visible differences between chimp and human chromosomes are few and tiny. He notes that if you select at random any ‘paragraph’ in the chimp genome and compare it with the comparable ‘paragraph’ in the human genome, you will find very few ‘letters’ are different: on average, less than two in every hundred. He also states that humans are more chimpanzee-like than gorillas are, and that there is no bone in the chimpanzee body that he doesn’t share. He concludes that there is no known part of the immune system, the digestive system, the vascular system, the lymph system or the nervous system that humans have and chimpanzees don’t or vice versa.
Ridley states that the common ancestor of humans and chimpanzees existed around 5-10 million years ago. He also mentions that there are very few differences between the DNA of humans and chimpanzees, and that most of the physical differences between the two species can be explained by changes in their pelvis and brain size. He also mentions that the missing link between the two species probably looked more like a chimpanzee than a human, and that the evolution of humans and chimpanzees is still ongoing.
Ridley states that calibrating the molecular clock to give an actual date in years is much more difficult for apes. Because apes are long-lived and breed at a comparatively advanced age, their molecular clocks tick rather slowly (the spelling mistakes are picked up mostly at the moment of replication, at the creation of an egg or sperm). However, it is not clear exactly how much to correct the clock for this factor; nor do all genes agree. Some stretches of D N A seem to imply an ancient split between chimps and human beings; others, such as the mitochondria, suggest a more recent date. The generally accepted range is five to ten million years. Apart from the fusion of chromosome 2, visible differences between chimp and human chromosomes are few and tiny. In thirteen chromosomes no visible differences of any kind exist. If you select at random any ‘paragraph’ in the chimp genome and compare it with the comparable ‘paragraph’ in the human genome, you will find very few ‘letters’ are different: on average, less than two in every hundred. We are, to a ninety-eight per cent approximation, chimpanzees,\and they are, with ninety-eight per cent confidence limits, human beings. If that does not dent your self-esteem, consider that chimpanzees are only ninety-seven per cent gorillas; and humans are also ninety-seven per cent gorillas. In other words we are more chimpanzee-like than gorillas are.
Matt Ridley explains that by analyzing the genetic record of humans and chimpanzees, scientists can gain a better understanding of the evolution of the two species and the pressures that shaped them. He argues that the differences in the genome, specifically in the genes that regulate growth and hormonal development, will provide valuable insights into the ecological and social evolution of the two species and the pressures that shaped them during the Pleistocene era. He suggests that the genetic record is a more valuable source of information on our past than traditional historical records.
Matt Ridley uses photography as an analogy to explain how the genes of an organism determine its development and behavior. He explains that while the genes act as a “negative” that contains the instructions for the development of an organism, the environment or “womb” acts as a “developer” that brings the instructions to life. He then goes on to explain that while genes determine the physical characteristics of an organism, they also play a role in determining its behavior. He acknowledges that the exact way in which genes influence behavior is not fully understood but is certain that it does.