If you’re a nature-lover or have binged a few David Attenborough’s Planet Earth documentaries, you’ve probably pondered the existential question of creation at least once. What makes us human? Are we humans a special species, or are we merely more refined and intelligent than our cousins in the jungle? For centuries, scientists and anthropologists have debated whether humanity was merely a lucky roll-of-the-die of Darwin’s Law, or whether our existence was brought into being by a superhuman entity. A recent development in biological anthropology has been made that could, once and for all, answer the question we’ve all been asking – what makes humans so human?
To understand the process that has taken billions of years to create a species like humans, one must first start at the molecular level. The human genome, our DNA sequence, contains 3.2 billion nucleotides, the building blocks of the genetic code of life. Some nucleotides are used by cells to construct proteins, ranging from regulating the production of the lactase enzyme to providing the body the energy it needs when sprinting. Other parts of the DNA molecule, noncoding regions, can act as regulators of protein synthesis, controlling when and how these proteins are produced. Yet, as it happens, about 90% of DNA appears to never be translated into these proteins. This enigma has stumped scientists for decades: Why does our genome contain so much “junk” DNA?
In the past, theories have been made about how the junk DNA may serve to “absorb” genetic mutations, in order to protect us from genetic diseases like sickle cell anemia. When mutations occur in noncoding DNA regions, this would leave intact the cellular processes vital to life. However, this theory is backed by no real evidence. However, a much more compelling argument was brought forth when researchers at UC Davis explored the genetic differences between DNA regions in humans and other primates. What they discovered stunned the scientific community: At the molecular level, the greatest differences exhibited were found in these very noncoding regions.
To fully appreciate the implications of these differences, one must first understand that the noncoding regions of the chimpanzee genome have remained unchanged for millions of years. This rigidity may be in part explained by natural selection. Although their genes weren’t as “intelligent” as those of humans today, the right combination of them allowed chimpanzees to survive well in jungles. That was all that mattered. Most chromosomal mutations that occurred left the mutant animals dead – that is, until a specific novel variant emerged containing precisely the right DNA favoring its survival. Scientists hypothesize that it was this genome, containing the DNA sequence that makes us human, that lives in our cells today.
By comparing the differences between gene groups and performing gene ontology analysis* to determine their functions, it was determined that the vast majority of these genes didn’t code for proteins themselves, but rather played significant roles in the regulation of certain processes involving brain function and development. This may explain why we have termed so much of our DNA as “junk” to begin with; although the vast amount of our DNA may not directly translate into proteins, it merely provides the scaffolding and mechanisms for other portions to do so.
Not only do the noncoding regions of our DNA explain the differences between us and our ancestors, but they pave the way for possible treatments for psychiatric disorders. In these regions, a single nucleotide polymorphism, or the replacement of one nucleotide base for another at a particular location, has been correlated with the development of autism. Furthermore, similar disorders like schizophrenia can be caused by mutations in genes fundamental to brain functions. What’s more, these disorders are not present in chimpanzees, corroborating the notion that noncoding regions are what make humans unique in terms of brainpower. Armed with this knowledge, if doctors are able to diagnose patients by examining their DNA, this may lead to a medical revolution in diagnosis and eventual treatments of such disorders.
Returning to the evolution of humans from chimpanzees, our differences in noncoding regions may explain why we are so different. Perhaps, if nothing else, this discovery sheds light on the evolution of humans, a remarkable process that has taken hundreds of millions of years to occur. Because of our enhanced ability to regulate the genes associated with brain development, intelligence, and problem solving, we were able to progress ahead as a species.
Finally, we may soon say that the human brain has been so intelligent as to have unraveled its own mystery.
* Gene ontology is the process of using a software program to analyze the biological processes a group of genes is involved with.