Dawn of the Planet of the Humans
By Raymond Huang
“If we evolved from monkeys, why are they still around?” This is an age old counterargument to Darwin’s natural selection, but it is as misleading as it is irrelevant.
Humans and other primates descended from a common ancestor 11 million years ago. Humans didn’t come from gorillas or chimpanzees, but they are closely related. The next question to ask is, if humans are so closely related to gorillas and chimpanzees, then why are humans the dominant species on this planet, and not any other primates? The answer lies in the genetic differences between the primates.
Humans, chimps, bonobos, gorillas, and orangutans belong to the Great Apes, characterized by larger brains and bodies than the Lesser Apes. Humans share more DNA with chimpanzees and bonobos, and we both play, have complex emotions, and have similar physical attributes. Pioneer primatologist Jane Goodall studied chimpanzees in Gombe and observed that the chimps there made and used tools, a trait that was once thought to separate humans from other chimpanzees.
But why humans? What separates Homo sapiens so far from their closest evolutionary cousins? You’re looking right at the answer. At the first level, you’re using an electronic device, a highly advanced machine with multiple underlying mechanisms that make this work such as the Internet and electricity. Next, you’re reading something written by me. You’re able to understand what I am writing because we speak the same language, complete with rhetorical devices, grammar, and references to past scientists Jane Goodall and Charles Darwin. These differences are a result of but a drop of water in the vast ocean of the DNA inside their cells.
Humans walk upright on two feet, which means their pelvises are straighter and narrower than that of other apes. This restricts childbirth because there is less room for babies to exit. The human brain is also at its maximum size at childbirth, any size larger would make it impossible for the child to exit. The evolution of these two traits are part of the Obstetrical Dilemma, which states that because of the narrower geometry of human pelvises, humans exit the womb early in their development because of their large head size. This also explains why human babies are helpless when born and why childbirth is painful and risky.
Human brains still grow to a remarkable size, twice as large during their first years of life and continues growing throughout their youth. This is because of the underdevelopment of their skulls. Gaps between skull plates are larger and fusing of the plates occur at a slower rate in humans than that of newborn chimpanzee skulls, which gives more time for the human brain to grow.
The neocortex is the layer on top of the brain that is responsible for higher cognitive functions. Compared to other mammals, human brains are very large for their body size. Compared to other apes, however, the neocortex size of humans isn’t much larger. Human brains also display deeper and more frequent convolutions -- foldings in the surface. This allows for a higher surface area of the brain, without affected the volume, which means more connections between neurons.
Humans and chimpanzees have an approximately 1.2% difference in DNA, excluding insertions, deletions, and duplications, and this 35 million base pair difference results in humans being the dominant race in the world. However, the difference between humans and other primates isn’t so much that different genes are being expressed, but rather different levels of gene expression. A study conducted in 2003 by Mario Cáceres found that elevated gene expression levels separate human brains from non-human primate brains. Approximately 90% of genes shared between the primates are more highly expressed in humans. The increased expression levels could be the cause of the extensive modifications of human brain function, physiology, and elevated levels of neuronal activity.
Ronny Stahl and Götz found that Trnp1 is an essential protein involved in mammalian brain development. The cortex is the outer layer of the brain and plays a key role in memory, attention, language, and much more. In their experiment, Stahl and co. demonstrates that high levels of Trnp1 promote tangential expansion of the cortex while low levels promote radial expansion and folding. Normally, mice have smooth brains, therefore high levels of Trnp1, but when mice had decreased levels of the protein, their brains developed convolutions similar to human convolutions. Stahl and his team also found regional differences in Trnp1 regulation in human fetuses, which show that the dynamic regulation of Trnp1 is crucial to control the expansion and foldings of mammalian cortexes.
The advancements in the human brain gives rise to their intelligence; their social skills, bone structure, and even the way they walk and use their hands create far more advanced communities of humans. The tiny, seemingly insignificant 1.2% difference in DNA determined what makes humans the dominant species on the planet. Their brains, their anatomy, separates them from their evolutionary cousins, and the gap is large and ever expanding.