Part 3... The Evidence


We have seen how physical traits, like specialized teeth in bears, can be traced through the fossil record from extinct species to modern bears. The fossil record for bears shows a very gradual evolution, consisting of a linear progression from one species to the next. But this is a rare case in evolution. Instead, many species’ evolutionary past can be described as a “tree”, with many branches and dead ends, rather than a single straight line.[1] This many-branched “tree” analogy applies to the evolutionary history of our species as well.[2]

Modern humans belong to the genus Homo, which includes us (Homo sapiens) and all of our immediate extinct relatives. Consider that this is the first time in human history where only a single species of humans occupied the planet at any one time. After the "hobbits" of Flores (H. florensis) and the last of the Neanderthals (Homo neanderthalensis) died out, about 18,000 to 28,000 years ago, respectively, we modern humans (Homo sapiens) became the last surviving human species.[3] Since then, we have spread all over the globe and have become one of the most dominant species on this planet. But how did we evolve?

As we discussed earlier, evolution is a theory of descent with modification. It implies common descent among organisms, living and extinct.[4][5] In terms of human origins, the theory implies that we are descended from an earlier ancestor who looked slightly different from us, because we’ve been “modified” in the course of our descent.

Figure 1. Descent with Modification: Your family tree illustrating your common ancestry with your cousins.

 

According to this “family tree” relationship, each of us is descended from our parents, who look slightly different from us, because we’ve been "modified" - or we’ve inherited half of our genes from each of them. Our parents in turn descended from their parents, who also looked slightly different than them. Their parents descended from their parents, and their parents form theirs, and so on.

In this family tree, you are related to your cousins because you share common ancestors - your grandparents. However, this tree doesn’t stop at your grandparents, or great grandparents. The human lineage spans much further back in time.

Evidence from the fossil record and DNA studies has shown that the first modern humans (H. sapiens) appeared in eastern Africa roughly 200,000 years ago.[7] All of us can trace our mitochondrial DNA to a single woman, a hypothetical "Eve," who lived in Africa at that time. All modern humans alive today are genetic descendants of this hypothetical Eve.[8]

In fact, using genetic markers, Spencer Wells of the National Geographic’s Genographic Project, has been able to follow our lineage from modern humans, all the way back to our “out-of-Africa” roots. “DNA studies suggest that all humans today descend from a group of African ancestors”[9] This small population first appeared in Africa about 200,000 years ago, then migrated out of the continent about 60,000 years ago, and colonized most of the planet. Everyone alive today is a descendant of this small group of people.[10]

Not only can we trace our ancestry to a hypothetical Eve living in Africa some 200,000 years ago, but we can trace our human lineage even further back in time to a common ancestor of all Hominids (humans, the great apes, and our exinct relatives).[11] In 2005, Geneticists from the Chimpanzee Sequencing and Analysis Consortium revealed that we share about 98% to 99% percent of our DNA with our chimpanzee cousins.[12]

By comparing the two genomes, and analyzing the number of mutations that had built up since we split from the chimpanzee lineage, geneticists were able to calculate that humans and chimpanzees last shared a common ancestor between 4.6 to 6.2 million years ago.[12.1] Geneticists were even able to identify the exact points along the genome where the split occurred, as the video below from PBS's NOVA series explains.

 

Humans are genetically more closely related to chimpanzees then to any other living species. “Chimpanzees and humans differ by just over one percent of DNA, and there are striking similarities in the composition of the blood and the immune responses. In fact, biologically, chimpanzees are more closely related to humans than they are to gorillas.”[13] In other words, chimpanzees are more “human” than "ape." The human family tree has been recently revised to include humans and chimpanzees in a sub-tribe called “Homini,” in order to distinguish chimpanzees from the other great apes – gorillas and orangutans.[14]

Not only are chimpanzees genetically similar to humans, but they also display similar behavior as us. According to the Jane Goodall Institute, chimpanzees are, "capable of intellectual performances once thought unique to humans.”[15] In the wild chimpanzees conduct diplomacy and wage war. “In Uganda, John Mitani of the University of Michigan observed chimp patrols regularly policing the forest boundaries of their communities. One patrol was seen assaulting an adult male.”[16] In other instances, “After fights between two chimps, scientists said, others in the group were seen consoling the loser and acting as mediators to restore peace.”[17]

Chimpanzees are also, “capable of sophisticated cooperation in hunting. They use more tools for more purposes than any other creatures except ourselves. And they show the beginning of tool-making behavior."[18] At Gombe in Tanzania, for example, “chimps with stick tools are accomplished extractors of termites from their nests [while] at Bossou... chimps have learned to make use of many other tools, including stones for cracking nuts.”

This behavior also reveals that chimpanzees have, "cultural differences from one chimpanzee group to another... Some of the activities are common to all communities, such as inviting others to play by holding a stem in the mouth. Other behaviors depend strictly on local conditions, including fishing with a stick for edible algae that's only present in specific locations."[19]

"In captivity chimpanzees can be taught human languages such as American Sign Language. They can master many complex skills on computers.”[20] They even outperform their human counterparts in memorization tasks.[21] “It has been demonstrated that chimpanzees are capable of reasoned thought, abstraction, generalization, symbolic representation and a concept of self."[22] These close genetic, anatomical, and behavioral relationships offer clues about our common descent with chimpanzees and the other great apes.

When Charles Darwin first published his theory of common descent in 1859, there was little physical evidence to support the notion that humans shared an extinct common ancestor with other great apes. The extinct Neanderthal lineage had just been discovered, but the fossils offered little insight into human evolution. The theory of common descent still lacked the “link,” or the transitional species, between modern humans and other members of the hominidae family.[23]

But today we know of over 20 extinct species of hominids that share physical characteristics of both modern humans and the other great apes.[24] A partial list of the hundreds of fossils of some of these extinct hominid species can be found on Michigan State University's website, located here. G. J. Sawyer’s “The Last Human: A Guide to Twenty-Two Species of Extinct Humans” also includes a thorough description of these extinct hominids. The most famous of these is “Lucy.”

Donald Johanson discovered Lucy’s fossilized remains in 1976 at Hadar in Ethiopia.[25] Through radiometric dating, we know that Lucy (Australopithecus afarensis) lived about 3.2 million years ago and had the physical characteristics of a chimpanzee,[26] with one very important exception - her pelvis looked more human-like than ape-like. This human-like pelvis exists only in the Australopithecus/Homo lineage. This is how paleontologists know that Australopithecines, including Lucy, were the common ancestors of all Homo, including modern humans. We all share Lucy’s distinctly “human-like” pelvis.[27]

 


Figure 2. Comparison of a modern chimpanzee pelvis with an Australopithecine and an early human pelvis.

 

We also know that Lucy’s unusual pelvis allowed her to walk upright. Unlike a chimpanzee, Lucy was bipedal, or walked on two legs. The evidence comes from footprints preserved in volcanic ash, which were found at Laetoli in Tanzania, and dated to roughly 3.8 to 3.5 million years ago. They were attributed to Australopithecines, like Lucy, who lived at that time.[28]

Similar to the evolution of novel traits from our earlier examples, such as the specialized teeth in bears, this unusual pelvis, which allowed Australopithecines like Lucy to walk upright, was so successful that it was handed down from generation to generation, all the way to modern humans. Just as we can trace the evolution of bears through the fossil record by following changes in their teeth, we can trace our own evolution by following skeletal changes in the fossilized remains of earlier humans. These changes are not only evident in the Australopithecine pelvis, but also in their skull and brain cavity.[29]

Through the course of human evolution, there has existed a trend to increase brain size.[30] We find smaller and smaller brain cavities among hominid fossil remains the further back in time we go. In fact, the brain cavity in the earliest hominids, the Australopithecines like Lucy, was only scarcely larger than that of a chimpanzee. (As we noted earlier, except for her pelvis, Lucy had the characteristics of a chimpanzee, including comparable brain size.)[31]

 


Figure 3. Comparison of a chimpanzee skull with an early hominid (australopithecine) and a late hominid (human) skull. Red arrow indicates change in brow ridge.

 

The ability to walk upright and the tendency for larger brains in human evolution is reminiscent of our earlier example from the evolution of polar bears. If you recall, when brown bears became trapped in the Arctic, their environment favored bodies, which were better adapted to surviving in their new environment. Likewise, adaptation to a changing environment among hominid populations favored the ability to walk upright and drove the increase in brain size. [32]

We know that throughout the course of human evolution, eastern Africa underwent climatic change. The environment shifted from dense forests, to open grasslands, that now make up the African savanna.[33] Our ancestors’ survival in the savanna may have favored the ability to walk upright, since, “fragmentation of once continuous forest caused our ancestors to move on the ground for longer distances, requiring more rapid movement…”[34] This is often referred to as the “Savanna Theory” of the evolution of bipedalism.

As further climatic changes occurred, including rapid cycles of glaciation, they pressed hominids to adapt even further. For hominids, “a larger brain proved useful in adapting to changing conditions because it allowed the development of cooperation…”[35] The ability to use their large brains to cooperate, solve problems, and to plan ahead helped hominids survive.[36]

Just as we can trace the evolution of bears by following changes in their teeth to adapt to their changing environment, we can trace our own evolution through the fossil record by following the increase in the size of our brain cavity, [37] as we adapted to a new way of life in our ever-changing environment.

 


Figure 4. Change in brain cavity size among hominids, showing a general trend towards larger brains in human evolution.

 

This video from a National Geographic Channel special demonstrates the genetic mutations which allowed the human brain cavity to expand and triple in size in such a short period of time.

Evidence from the fossil record, DNA studies, and similarities in living species give us a better understanding of the evolution of mankind. Changes in skeletal structures, including the pelvis, brain cavity size, and other changes in the skull, as well the age and geographic location of fossil remains allow us to construct a more complete tree of human evolution. Although there are many branches and dead ends in this tree, we can begin to recognize the evolutionary relationships between apes, the many different extinct species of hominids, and us.

 


Figure 5. Temporal relationship for known hominids.

 

The fossil record reveals that Hominids diverged from a common ancestor with chimpanzees about 6 to 8 million years ago; But our family tree doesn't stop there. If we look even further back in time in the fossil record, we will find that all hominidae, have descended from a common ancestor of all primates.[38]

Figure 6. Evolutionary Tree for Primates.

 

Similarly, we can trace our lineage even further back, to the common ancestor of all placental mammals. And if we look further back in time in the fossil record, we will find Tiktaalik, a fish with wrist bones, who may have been the common ancestor to all tetrapods (four-legged vertebrates), including us.[39] You can read about Tiktaalik and the remarkable similarities this fish had to modern humans in Neil Shubin's book, Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body.

In these few pages we described what evolution is and how it works. In our ladybug example, we saw that evolution relies on variation among populations. In our example from pesticide resistance within amaranth populations, we saw that natural selection, the driving force behind evolution, exists in nature today. In our example from bear evolution, we saw how natural selection acted to create modern polar bears. And, from our last example of hominid evolution, we saw that the basic concepts of common descent apply to our species as well.

These few examples provide limited insight into the history of life on this planet. This website is intended to be a very basic introduction to the theory of evolution. The history of life on Earth is much more complex than what we can cover in these few pages. We encourage everyone to explore other resources and to read more in order to fully appreciate the tremendous diversity of life on this planet.

 

Further Reading

There is a large amount of reading material on human evolution. Below is a very limited list of the more popular general interest books. A great introduction to human evolution is Carl Zimmer’s Smithsonian Intimate Guide to Human Origins.

How We Know What We Know: A Case Study in Human Origins.”

National Geographic. "The Genographic Project: Human Migration, Population Genetics, Maps, DNA." 1996-2008 National Geographic Society

Pollard, Katherine S. "What Makes Us Human?" Scientific American. May 2009

Wilford, John N. "Almost Human, and Sometimes Smarter" The New York Times. April 17, 2007

 

 

 

References

[1] Dugger, William M. Sherman, Howard J. Evolutionary Theory in the Social Sciences Taylor & Francis, 2003. p. 37-38

[2] Kolb, Bryan; Whishaw, Ian Q. Fundamentals of Human Neuropsychology Macmillan, 2008. Chapter: “Hominid Evolution,” p. 35-36

[3] Zimmer, Carl. Smithsonian Intimate Guide to Human Origins Collins, 2005

[4] Ridley, Mark. Evolution. Wiley-Blackwell, 2004. p. 4-5

[5] Carroll, Joseph; Darwin, Charles. (Carroll, Joseph editor) On the Origin of Species by Means of Natural Selection. Broadview Press, 2003. p. 14; Dupré, John. Darwin's Legacy: What Evolution Means Today. Oxford University Press, 2005, Eldredge, Niles. The Triumph of Evolution. Macmillan, 2001. p. 25-26

[7] Johanson, Donald C. From Lucy to Language. Simon and Schuster, 1996. p. 41

[8] Zimmer. p 106; Johanson, p. 41

[9] National Geographic. "The Genographic Project: Human Migration, Population Genetics, Maps, DNA." 1996-2008 National Geographic Society.

[10] Kolb, p.35-36

[11] Johanson, p. 41

[12] Zimmer, p.34 (genetic similarity between humans and chimpanzees varies from 96% to 99%, depending on sources) Original paper: Chimpanzee Sequencing and Analysis Consortium."Initial sequence of the chimpanzee genome and comparison with the human genome." Nature. 2005 Sep 1; 437 (7055):69-87.

[12.1] Chen, Feng-Chi; Li, Wen-Hsiung. "Genomic Divergences between Humans and Other Hominoids and the Effective Population Size of the Common Ancestor of Humans and Chimpanzees." American Journal of Human Genetics. 2001 February; 68(2): 444–456.

[13] The Jane Goodall Institute. 2009.

[14] Steele, Fintan R. "Human and chimp genomes reveal new twist on origin of species" Broad Institute of MIT and Harvard. May 17, 2006

[15] Goodall.

[16] Wilford, John N. "Almost Human, and Sometimes Smarter" The New York Times. April 17, 2007.

[17] Wilford.

[18] Goodall.

[19] Cromie, William J. "Chimpanzee Behaviors Surprise Scientists." The Harvard University Gazette. June 17, 1999. President and Fellows of Harvard College. 1999.

[20] Goodall

[21] Wilford.

[22] Wilford.

[23] Wilfrid E; Le Cros, Clark. Man-Apes or Ape-Men? The Story of Discoveries in Africa Holt, Rinehart and Winston, 1967. p. 150

[24] Sawyer, G. J; Deak, Viktor; Sarmiento, Esteban; Milner, Richard. The Last Human: A Guide to Twenty-Two Species of Extinct Humans. Yale University Press, 2007

[25] Johanson, D. C. and Taieb, M. “Plio-Pleistocene hominid discoveries in Hadar, Ethiopia.” Nature 260, 1976. p. 293–297

[26] Zimmer.

[27] Johanson, 1996, p. 86-88

[28] Leakey, M. D. and Hay, R. L. “Pliocene footprints in the Laetolil Beds at Laetoli, northern TanzaniaNature 278, 1979. p 317–323; Johanson, 1996, p. 87

[29] Wood, Bernard A. Human Evolution. Taylor & Francis, 1978. p. 40

[30] Wolpoff, Milford H. Race and Human Evolution. Simon and Schuster, 1997. p. 234-235

[31] Wade, Nicholas. Before the Dawn: Recovering the Lost History of Our Ancestors. Penguin, 2006. p. 18

[32] Wade, p. 7

[33] Ochoa, George; Hoffman, Jennifer Ruth; Tin, Tina. Climate: the Force That Shapes Our World and the Future of Life on Earth. Rodale, 2005. P. 54, and p. 56; Ruddiman, p. 384

[34] Ruddiman, William F. Earth's Climate: Past and Future. Macmillan, 2001. p. 384

[35] Ochoa, p 60

[36] Kappeler, Peter M. Cooperation in Primates and Humans: Mechanisms and Evolution. Springer, 2006. p. 232

[37] Schwartz, Jeffrey H.; Holloway, Ralph L; Broadfield, Douglas C; et al.The Human Fossil Record: Brain Endocasts: the Paleoneurological Evidence. Wiley, 2004. p. 15-17

[38] Fleagle, John G. Primate Adaptation and Evolution. Academic Press, 1999

[39] Shubin, Neil. Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body. Vintage, 2009

 

Image Credits:

Figure 2. Comparison of a chimpanzee pelvis with an early hominid (australopithecine) and a late hominid (human) pelvis. Based on: Zihlman, Adrienne L; Simmons, Carla. The Human Evolution Coloring Book. HarperCollins, 2001.

Figure 3. Comparison of a chimpanzee skull with an early hominid (australopithecine) and a late hominid (human) skull. Based on: Zihlman, 2001

Figure 4. Change in brain cavity size among hominids. Redrawn and Translated from: "Volume cérébral des Hominidés". Le Journal duNet.

Figure 5. Temporal relationship for known hominids. Source: Sawyer, G. J, et al. The Last Human: A Guide to Twenty-Two Species of Extinct Humans. Yale University Press, 2007

Figure 6. Evolutionary Tree for Primates. Source: Fleagle, John G. Primate Adaptation and Evolution. Academic Press, 1999