The Human Tree of Life: Did We Evolve from Chimpanzees?

https://www.history.com/news/humans-evolution-neanderthals-denisovans

We have all seen this picture at some point in our lives. Whether it is this exact photo or one that begins with a fish crawling out of the water, we have all at one point been taught that evolution followed a straight line, ending in humans today. As if we are the end: the peak of evolution. So did we really evolve from chimpanzees, as is the common theme in these pictures? I hate to be the bearer of bad news, but believing this is as ignorant as believing that the Earth is the center of the universe. People love to assume that evolution occurs from species that we see today, assuming that the same fish we see in our oceans today crawled out of the water evolved along a cascade of present animals until chimpanzees finally evolve into humans. To them, evolution is the change in a species over time, and changing species simply means becoming different animals. Evolution just isn’t that simple. Humans did not evolve from chimpanzees; we actually evolved together.

For as long as people have developed an understanding of the world around us, we have attempted to find our place in it. For thousands of years, people told stories of heroes and epic tales through constellations in the stars, unknowing that they were in fact enormous nuclear reactions light years away. We told tales of monsters who would eat sailors who sailed over the edge of the ocean, unknowing that we lived on one of many planets orbiting the Sun. Whatever we saw in front of us, we assumed that we were at the center of it, not a part of it.

We had the same beliefs about our origin as a species, with the Roman Catholic Church forcing the belief of divine creationism on its followers. A belief that only ended when a man by the name of Charles Darwin took a boat trip around the world. In the early 19th century, Darwin landed in the Galapagos Islands off the coast of Ecuador, where he studied the differences between 15 species of finches (birds that are closely related to cardinals and sparrows). By studying the differences in physical characteristics and behaviors, Darwin was able to propose his theory of evolution¹. The crown jewel to Darwin’s theory was natural selection, or as most people might have heard it, “survival of the fittest”. According to Darwin, evolution is not possible without it.

Though most people have heard it, few interpret it for what it is. Natural selection allows individuals with beneficial traits to live on and produce offspring with the same beneficial traits. Unfortunately, organisms that do not have these traits die off, not passing their genes onto the next generation. Though natural selection usually takes place gradually over millions of years, there have been exceptions in nature that have accelerated the process up to the point where we are able to observe a noticeable transformation. Take the tiny Rock Pocket Mouse, native to the deserts of New Mexico and Arizona. Volcanic eruptions just over 1,000 years ago have left large patches of black volcanic rock scattered throughout the light sand. As predators were more easily able to see light-colored mice on the rocks, they were hunted out in areas covered by the volcanic rocks. The darker-colored mice, who were unseen by the predators, were allowed to reproduce, until all pocket mice living on these volcanic rocks had dark fur².

Animal populations and hominid populations alike adapt and change over time to particular challenges, whether it be developing a mutation that allows for disease resistance or a physical attribute that allows it to be better suited for its habitat. In this sense, humans have begun to play God. Due to our ever-accelerating development of new technology, we have been able to deal with our adversities through ways other than evolution. In this sense, natural selection doesn’t apply to modern humans in the same way that it applies to other species. Take a person with 20/400 vision. What would have lead to almost certain death (either by injury or attack from predators) 100,000 years ago can be fixed with a pair of prescription glasses. Modern medicine has eradicated smallpox, a virus responsible for the death of over half a billion people throughout history. Access to such advanced technology has lead to the belief that natural selection isn’t happening to us at all. As if we had reached a climax, despite natural selection happening spontaneously to us and all other organisms. Natural selection has been a characteristic of living organisms ever since the beginning of life on Earth and along every step of the way. It was natural selection that led to the first multicellular organisms. It was natural selection that led to the first aquatic organisms making homes on terrestrial landscapes. And it will be natural selection that fuels any unstoppable changes to the human population. Now, when did this all begin?

All life on Earth started over 3 billion years ago with LUCA, or the Last Universal Common Ancestor. Every organism, alive or extinct, has evolved from LUCA, continuing to further evolve into the millions of confirmed species on the planet today. We aren’t further along the branches of the tree of life than any particular organism, we are just on our own unique branch. Just as real trees have branches that merge together into a singular trunk, the tree of life is much of the same way. Called phylogenetic trees by biologists and ecologists, these trees of life start out with a single common ancestor. This depends on the tree that the scientist is making, ranging from the first bipedal apes to the first mammals, even LUCA itself. On the tree of life, humans have evolved separately from chimpanzees over 7 million years ago (mya)³, and this is the great ape with the most recent common ancestry to modern humans. When looking at gorillas, the evolutionary split occurred much earlier, as much as 11 million years ago⁹. So much for evolving from chimpanzees, but the question still remains. How did modern humans come to be?

Scientists were mostly in the dark on the processes of the evolution of modern humans. That all ended on one October day in 1974.

Lucy, named after the popular Beatles song “Lucy in the Sky with Diamonds”, was a fossil of the species Australopithecus afarensis found in the Afar region of Ethiopia. Dating back to almost 3.2 million years ago, Lucy still stands today as the oldest hominid fossil ever discovered. What made this discovery so important, aside from being the first fossil remains of the species found, was the characteristics that it shares with both modern humans and chimpanzees. Along with having an elevated brow line and flat nose, Lucy exhibited traits that allowed her to walk upright, a process called bipedal walking. Along with using tools and fire, bipedal walking was one of the key features that separated hominids from other apes. Lucy was all it took to convince University of California Berkeley professor Tim D. White that, although not definitive, Australopithecus afarensis was the probable direct ancestor of modern humans⁴.

What’s most amazing is that Lucy wasn’t walking alone. Through studying bone fragments of early hominids, scientists slowly started growing the list of bipedal hominids. These included species such as Australopithecus anamensis, Orrorin tugenensis and Ardipithecus ramidus, with the latter two believed to have roamed eastern Africa as early as 6 million years ago⁵. The timeline of these fossils, being before the divergence date of hominids and other great apes, means that bipedal walking is a trait that evolved only in the hominid branch of the tree of life. It is important to realize: around the same time, apes were roaming the Earth that were the distant ancestors of modern chimpanzees. Like Lucy with humans, these apes probably have characteristics of both other apes and chimpanzees.

Ever since the discovery of Lucy, scientists have been slowly trying to piece together the complete fossil history of human evolution. Even though dozens of hominid species have been identified, scientists are still discovering unfound remains, even unfound species. Take Homo naledi. Undiscovered until 2013, this specimen was found in a rich fossil site in South Africa and showed ape-human hybrid characteristics. Sounds familiar right? Just another Lucy? Well, you would have to wait another 2.7 million years after Lucy to see a living Homo naledi. What truly surprised scientists was where exactly they found these fossils: in a cave chamber deep underground that requires squeezing through a 7 inch gap in order to access. Why would primitive apes make the effort to go down into these caves anyways? What scientists believe is truly mind-boggling: they believe they were disposing of their dead. Though its discovery has further complicated the full understanding of human evolution, it is clear that it has a definitive place in the hominid tree of life⁶.

Though a definitive human phylogenetic tree is impossible with the data we have obtained today, it is possible to come up with theoretical models. Since models are only based on fossil records, divergence dates and extinction dates between species are approximations given by scientists about the ages of their specimens.

Phylogenetic tree of various great ape and hominid species over the last seven million years. Horizontal lines represent a species through time, with the dashed end representing its extinction or current existence. Out of the 26 total species on the tree (and labeled numerically on the list below), only Gorilla gorilla, Gorilla beringei, Pan paniscus, Pan troglodytes, and Homo sapiens are alive today. Though it may be unclear on the phylogenetic tree, Homo neanderthalensis went exist alongside modern humans around 20,000 years ago⁷*⁸*⁹*¹⁰*¹¹*¹².

It took millions of years for primitive apes to evolve into modern humans. Dozens of species came and went, and we stand alone along with the modern descendants of the divergence that took place 7 million years ago. What does that mean for humans now? Well, it surely doesn’t mean that evolution is going to slam on the breaks anytime soon. Modern humans first existed around 200,000 years ago, and have slowly spread to inhabit all continents (except Antarctica). As human populations diverged, particular genes in DNA underwent mutations, or changes to the DNA sequence that drive evolutions. By looking at the sequences of 29 genes, scientists were able to create a phylogenetic tree of human populations. What they found was populations such as Nigerian, Pygmy, Bantu, and San (all populations native to the African continent) were evolutionarily separate from all other populations, indicating that evolution between populations occurred as humans migrated out of Africa. Further supporting this concept, another major divergence soon took place afterwards, where Middle Eastern and European populations diverged from populations native to Asia, Oceania, and the Americas.

It is a shame to see such a wonderful process of nature go misunderstood. Human intelligence has allowed us to find our place in the universe, while human ignorance has served to halt our progress for as long as human civilization has been. Perhaps with more discoveries in caves in Ethiopia, Kenya, and South Africa, we will discover more undiscovered hominids and early apes, un-cracking the evolutionary pathway that we have only been able to put together with uncertainties. As the research continues we can finally piece together our past to help us through the future.

1. Lamichhaney, S. (n.d.). Retrieved April 12, 2020, from https://scholar.harvard.edu/sangeet/adaptive-evolution-darwins-finches
2. The Making of the Fittest: Natural Selection and Adaptation. (n.d.). Retrieved April 12, 2020, from https://www.biointeractive.org/classroom-resources/making-fittest-natural-selection-and-adaptation
3. Langergraber, K. E., Prüfer, K., Rowney, C., Boesch, C., Crockford, C., Fawcett, K., … Vigilant, L. (2012). Generation times in wild chimpanzees and gorillas suggest earlier divergence times in great ape and human evolution. PNAS, 109(39), 15716–15721
4. Hogenboom, M. (2014, November 27). The ‘Lucy’ fossil rewrote the story of humanity. Retrieved March 9, 2020
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10. Frandsen, P. (2019, October 6). Revealed: the ancient genetic link between chimpanzees and bonobos. Retrieved April 12, 2020, from https://theconversation.com/revealed-the-ancient-genetic-link-between-chimpanzees-and-bonobos-67760
11. Curnoe, D. (2018, July 12). When humans split from the apes. Retrieved April 12, 2020, from https://theconversation.com/when-humans-split-from-the-apes-55104