This post covers chapters 21 & 22 from Futuyma and Kirkpatrick’s book on Evolution (2018). The author of this post is Sam Shry.
Evolution and Homo sapiens; the birds and bees of where we come from
We now turn our attention to the evolution of our own species and how we came to be this unique, evolutionary phenomenon. Of course, we have already learned that we arose from a last universal common ancestor (LUCA), but closer in evolutionary time we arose from the group called great apes, more specifically our closest relatives are the chimpanzee (Pan troglodytes) and the bonobo (Pan paniscus). The lineage hominin (us) diverged from the chimpanzee lineage around 7 Mya, with Homo sapiens becoming the only species from this lineage not to go extinct. With genetic similarities in our protein-coding genes of more than 98% between chimpanzee and humans, it’s no wonder that there are such close similarities between these two species in terms of morphological, social, and cognitive features. We, however, differ in many aspects too, for example, we have evolved the ability to live upright on two feet (bipedal), adapting to a changing climate during this time via movement and migration. We also evolved reduced body hair to probably aid in making sweating and evaporative cooling more efficient. We also have many differences in our hand morphology, teeth, and our larger brain. What is interesting to me is that our potential growth rate is so much higher than for other ape species, as human females can become pregnant even when they have dependent children, in contrast to chimpanzees that cannot.
The divergence of hominins occurred in Africa, as Darwin predicted long before fossil evidence. Hominins split into several species, with all (except us) eventually dying out, giving an interesting insight into the evolution of our species. One of these (Ardipithecus ramidus) had apelike features like a chimpanzee’s brain size and climbing adaptations, but also had small canine teeth and a pelvis adapted for walking upright. The genus Homo dates back about 3 Mya, where the Homo habilis and Homo erectus are thought to be the ancestors of our own species. Homo habilis resembled modern humans closer than earlier species, with a flatter face, shorter tooth row, and greater cranial capacity than before. Homo erectus had a resemblance that was even closer to modern humans and was the first hominin to leave Africa around 2 Mya, spreading into the Middle East and eventually Asia. Our ancestors continued to slowly disperse across the world, with Homo heidelbergensis, a common ancestor to the well-known Neanderthals, dispersing throughout Europe and Asia around 600 Kya (figure 1). I like the analogy in the book that describes the phylogeny of hominins as a densely tangled bush, with humans only being a leaf on this bush.
We can map our ancestral dispersal across earth over time using our gene trees of mitochondrial DNA (mtDNA) sequences to complement and provide additional insight to fossil evidence and estimate the age of the most recent common ancestor of mtDNA in living humans. We also find that these different groups of hominins hybridized, for example, Neanderthals and Denisovans, contributing advantageous alleles to the human gene pool. In today’s human populations, we also find some genetic differentiation, but these differences are very low compared to other species, due probably to the very short amount of time we have evolved since colonization. What is surprising is how adapted some human populations are to their environments due to high selective pressure, for example, lighter skin color in Europeans and East Asians adapted to limited sunlight in northern latitudes.
One important characteristic of humans is our brain size. Our cognitive abilities far surpass other species, giving us the ability to reason, think, and understand. One hypothesis for this is ecological, in that we evolved cognition for dealing with complex environments. Another hypothesis is called the social brain hypothesis, where our complex social groups selected for large brains that would ultimately increase survival and the chance for reproduction (increasing fitness). Social cooperation and accumulated knowledge via the use of language are key elements to the evolution of increased cognitive abilities in our species. The evolved modification of our vocal tract enables vocal versatility, enabling a wider range of sounds, but at the cost of possibly choking to death on food, so the advantages of spoken language must be strong in our evolution.
These somewhat strange evolutionary tradeoffs are further seen when looking at the costs of a larger brain, causing higher reproductive costs and requiring more energy to grow all the way up to 18 years, making human life history paradoxical. We do however have longer life spans than other primates, we evolved higher metabolic rates, higher reproductive rates, and a larger energy budget. We did this by switching diets to adding meats and tubers, and learning to hunt and gather. Agriculture was developed and animal domestication began to feed our growing population. Agriculture was double-sided, providing food stability, but changing human lifestyle, culture, and environment around us. Adapting to a new diet selected for specific genes, for example, the lactase persistence enzyme that allows for the digestion of lactose in milk. What is also interesting is how our ancestor dispersal from Africa left traces in our DNA sequence variation, where individuals that disperse furthest from Africa have the highest deleterious mutations, given they were small populations (founder effect), accumulating rare variants, less effective natural selection, and intense drift that fixed these mutations.
Natural selection and evolution are still acting on our species, it’s just that the selection pressures have changed. We find modern hygiene and medicine alleviate the fitness differences, but still, we see selection occurring in some traits, such as cholesterol levels and height, due to our diet, lifestyle, and environmental factors. We also still see the effects of changing to an agricultural diet in our obesity crisis, with calorie-dense foods combined with our sedentary lifestyles. We also have the evolution of culture acting on our society, simultaneously driving cultural elements, innovation, and social interactions. The cumulation of cultural influences, human behavior, and biological tendencies guide, but do not limit our diversity as we continue to move forward in time.
Evolution and Society
The theory of evolution by natural selection has had a profound impact on our understanding of ourselves and the world around us, “unifying the realm of life, meaning and purpose with the realm of space and time, cause and effect, mechanism and physical law.” – Daniel Dennett
Although there is a long line of evidence behind this theory’s confirmation, there are still those that do not accept this theory as reality and cannot grip general reasoning. An extreme, sub-population have “beliefs” in creationism, that God directly created the human species. Certain, literal interpretations of the Bible and other religious documents have conflicting ideas of Earth’s creation and its life, for example in the book of Genesis and the literal interpretation of the creation of Earth and life in six days. Others believe evolution is the mechanism by which God enabled/s creation to proceed and now lets the Earth run on its own without further intervention. People can believe whatever they want, but issues arise when religion and beliefs are confused with science and education. For example, in the US, some schools are required to give equal teaching time to religious doctrine like intelligent design as they do to scientific theory such as evolution. The problem with this is that people do not understand how science works and how it is not comparable to religion. The process of science is to forever test our understanding of the world through the scientific method. Over time we have come up with our best understanding of the world and these denoted theories are “proven” beyond a reasonable doubt via scientific testing and evidence-based science. As this book has explained over the last 21 chapters, the theory of evolution is a highly complex system that we are continuing to understand better every day. Evidence for evolution has been already described throughout the book but we can just recap a few examples. The fossil record and the transitional fossils between species have given us physical evidence of evolution that often matches up, time-wise, with phylogenetic predicted sequences. The phylogenetic relationship between species via DNA analysis has opened a vast new world of understanding every species connection and diversity. Though this evidence does not waver for some true believers, the failed argument of intelligent design gives a baseless ground for these beliefs. The whole argument of a godly design is that these complex species are created perfectly. So, the vast number of failed, inferior designed traits or species in this world would indicate an unkind, incompetent designer. Why would an ultimate designer create sickle-cell disease, saving some and killing everyone else that is homozygous for a gene? Intelligent design is unable to explain the selfish behaviors that natural selection can explain, like cannibalism, siblicide, and infanticide. We have also been able to use the information gleaned from the mechanisms governing the theory of evolution to understand the world around us and how it is changing. For example, understanding domesticated plants and animals, resistance to pesticides and antibiotics, and rapid adaptations to climate changes. In short, the evidence behind evolutionary theory should never be compared to beliefs and even if they were to be, there is no evidence for these beliefs.
Understanding evolution and its mechanisms has shaped our understanding of the world today. It has formed the world we live in, ourselves, and every other species. The concept of natural selection can be applied to all aspects of our lives, such as culture, language, political science, and the overall human experience. The practical application of evolutionary mechanisms has helped us in advancing food production, managing natural resources, improving conservation, and human health. By having this understanding of how organisms evolve, we can make future predictions for our species, other species, and even understand how to develop new technologies and organisms to further benefit us or save our species. This work is vital to combating today’s challenges such as climate change, biodiversity loss, genetic diseases, cancer, infectious diseases, and general public health. One example is the current extinction crises, the global loss of species diversity. Due to rapid climate change, species are unable to genetically adapt to new climate conditions, causing species extinction. Evolution is a unifying theme throughout the biological and social sciences and is the backbone to understanding the world around us. It is, however, only knowledge, with no external meaning, morals, or ethics. It is important knowledge for our society, one species of the millions inhabiting this earth. It is always fun to think about how little evolutionary time our species has inhabited this earth and how/if we will endure or just become another species on the extinction list.