An Illustrated Guide to Human Evolution.

Every once and a while I’ll catch a glimpse of myself in the mirror, and I’m reminded of our not so distant ancestors.  My long arms and proclivity for climbing trees might make me seem more apish than your average Homo Sapiens Sapiens (the second Sapiens is because we’re modern humans) but we all come from the same common ancestor. Some 7 million years ago (ma) the earliest human predecessor diverged from the group that would become Chimpanzees and Gorillas. Changing climate, shifting ecology, and a host of other factors led to the selection of traits that we see as human. Over a multimillion-year journey we evolved to the upright, hairless, social media loving ape you see today.

                We can look back at the fossil record and see how our ancestors slowly changed. But studying the fossil remains of human ancestors is a little bit like trying to solve a million-year-old murder. Paleontologists have done incredible work with quite literally the barest bones of information. The most minute data ranging from the wear on teeth to the isotopes contained in bones informs our vision of the past. 

                There is a great deal of discussion regarding the implications of findings. Endless debate surrounds the latest discoveries, but discourse does not invalidate the theories on which we agree. It is only through the rigorous process of scientific analysis can the truth be fully grasped. I am going to stay away from more controversial theories, but I will mention all sides if I do.

                Specifically this paper will focus on the differences from one species to its successor and certain morphological adaptations that evolve into modern human traits. We will examine three parts of fossils: skull, teeth, and lower skeleton, to give us a picture of the past.

                A morphological trait itself cannot be modern or primitive. But when we compare traits of one species with another, we can identify trends. Chimpanzees are just as modern as human beings, but they do share similarities with our more primitive ancestors. So on one end of the spectrum is Chimpanzees (Pan Troglodytes) and on the other Human Beings (Homo Sapiens Sapiens).

Pan Troglodytes:

The differences between Chimpanzees and modern humans are representative of those between modern humans and our early ancestors. The Chimp skull, like many other Great Apes, has a low sloping forehead, a large brow ridge, and a lower face that juts out from the skull. The profile is immediately recognizable from the flat face of humans. The brain case is considerably smaller, approximately 300-450 cubic centimeters(cc), compared to the 1100 to 1200 cc for human beings. The jaw of the Chimpanzee is also larger. Another key difference is the foramen magnum. This large hole provides the location where the brain connects to the spinal cord and major blood vessels. In quadruped species the foramen magnum is located posteriorly (towards the back) on the skull. Compared to say a dog, which is a quadruped, the Chimpanzee foramen is anterior (forward), but compared to humans it is posterior. The location of the foramen magnum indicates whether a species is bipedal (walks on two legs) or quadruped (four legs).

                The teeth of the chimpanzee are robust compared to humans. Chimps have a great deal of space in their mouth because of their bigger jaw. The large canine teeth are separated from the others. The teeth are arranged in a ‘U’, with the two sides of molars parallel to each other, unlike humans which have more of a ‘c’ shape dental arrangement.

                Chimps are around 3ft-4.5ft tall (0.9-1.4m for our European Homo Sapiens), and display significant physical differences between the sexes (sexual dimorphism).  The lower skeleton is much more suited to climbing and traveling in tree branches (brachiation) and seldom terrestrial movement (walking around). The arms are longer than the legs and the fingers are curved, which helps with gripping branches. The feet as well are suited for gripping tree branches as opposed to walking. While chimpanzee’s legs bow outwards, humans have a knee that bends inwards, allowing us to easily place all our weight on one leg.

Australopithecus Afarensis:

Australopithecus was a diverse genus which contained many different species. But most researchers think that Australopithecus Afarensis was the specific species that gave rise to the genus Homo, from which humans arise. Living 3.9 to 3 million years ago (ma) A. Afarenis has more similarities with humans then other apes and existed after the point of divergence of the Chimp/Gorilla lineage. Even though it is not the juncture between humans and Chimps, A. Afarenis is a vital link that offers us a look at an intermediate stage.

                A. Afarensis’ skull is very ape-like with a sloping forehead, large brow ridge, and jutting jaw. The cranial cavity was around chimp size, 500 cc, much smaller than modern humans. The foramen magnum was located forward on the skull compared to Chimps, indicating bipedalism.

                The jaw and teeth of A. Afarensis are very interesting and somewhat controversial. The jaw structure is more like apes, the molars are parallel. But the teeth are less robust, like modern humans. The controversy arises because the teeth are shaped for eating tough brittle foods like nuts, but the actual wear and tear indicates that A. Afarensis were frugivores.

                The lower skeleton of A. Afarensis is much more human, however there is no definite consensus on how bipedal or arboreal the animal was. It had curved fingers like chimpanzees, and shoulders suited to climbing. But the bones we have of the legs suggest the A. Afarensis had a valgus knee like humans. A. Afarensis was between 3ft-5ft tall (0.9 to 1.5 m) and displayed sexual dimorphism. Males were generally a foot taller, which is much greater than sexual dimorphism in humans.

Homo Habilis:

Homo Habilis is the first member of the Homo genus, where we get the part of our name: Homo Sapiens Sapiens. It arose 2.4ma and went extinct 1.4ma.  Standing about 3-4ft tall (0.9-1.2m) H. Habilis had the largest brain case ,600cc, compared to body size of any ape so far and marks the first point in a trend of increasing cranial volume. There is also evidence that H. Habilis made use of stone tools. The face was still very prognathous, the forehead sloped, brow-ridge pronounced, and the jaw quite large. But all of these features were trending towards more human proportions.

                The teeth of H. Habilis were much more human and less robust. But thick enamel and powerful jaw meant that H. Habilis was still capable of chewing tough and brittle food. Markings on bones of animals found 2.6 ma indicate H. Habilis would butcher animals with primitive tools and consume the meat.

                Unfortunately there is not an abundant library of fossil specimens for H. Habilis. But what we do have indicates that it was a predominately bipedal species that pushed further into the savannah and away from the forest. It is a very rough example of the human form, but none the less it is the first of our kind.

Homo Erectus:

Arising 1.9 ma and extending to 110 thousand years ago(ka) Homo Erectus was truly recognizable as a human species. It had a cranial cavity that is quite large compared to its body volume. It was a species well suited to walking and running. Tool use also improved and the first example for the hand axe originated with H. Erectus.

                There is a rich sample of cranial specimens for H. Erectus. Because of this we see a great variation in morphology. But the general trend is a much flatter face and less robust jaw and teeth. The cranial capacity was around 840c, much larger than previous species, particularly in relation to its body size. The brow ridge was still very pronounced. The forehead sloped backwards as in contrast to the high dome of H. Sapiens Sapiens skull. The teeth are very modern in their arrangement and have thin enamel.

                The lower skeleton suggests a strictly bipedal lifestyle and is very similar to modern humans. The legs are longer than the arms, as opposed to chimpanzees with long arms and short legs. The height of H. Erectus varied greatly from below 5ft to 6ft tall (1.4-1.8m), and there was very little sexual dimorphism.

                Evidence suggests that H. Erectus made use of tools and engaged in hunting. It is possibly the first species to have made use of fire, a landmark in the development of technology. There is some indication that H. Erectus even constructed rudimentary shelter.

Homo Heidelbergnesis:

There is debate as to whether Homo Heidelbergensis is the direct ancestor of Homo Sapiens or if it is the predecessor of the offshoots that would become Homo Neanderthalensis. But I am going to consider H. Heidelbergensis either as the link between H. Erectus and H. Sapiens or at least indicative of the morphological intermediate.

                H. Heidelbergensis first arouse 700ka and went extinct around 200ka. It had height and weight quite similar to that of modern humans: 5.5-6ft (1.6-1.8m) for males and 5-5.5ft (1.5-1.6m) for females. The skull had a very robust brow ridge and jaw in comparison with Homo Sapiens but was diminished in respects to earlier species. The chin was receded and more chimp like, but the teeth were much smaller and modern. The cranial Capacity of around 1200 cc was very similar to that of modern humans at around 1100-1200cc.

                The lower skeleton of H. Heidelbergensis was tall, graceful, and well suited to a migratory lifestyle. The skeleton was similar to that of modern humans, save for the limbs which were more robust. They made extensive use of stone tools, were the first to use wooden spears, and definitely built and dwelled in shelters. Much like later species, H. Neanderthals and Homo Sapiens, the H. Heidelbergensis were regular hunters of large game.

Homo Sapiens, Neanderthals and Denisovans.

These three species of Homo all existed on planet earth around the same time. They were offshoots of a common ancestor and separated around 750,000 years ago. We are not descended from Neanderthals or Denisovans, but they give us a look at a parallel human species.

Homo Neanderthalensis:

Commonly referred to as Neanderthals this species was a large robust hominid. It is often confused as a missing link between us and the other great apes. But the Neanderthal was our contemporary before it went extinct. It was a human being not more primitive, but rather divergent.

                The Neanderthals had a larger cranial capacity than Homo Sapiens at around 1410 cc. The skull was particularly distinct from modern humans. The front of the face protruded, the chin receded, and the brow ridge was quite large. The skull, despite having more volume, sloped backwards. The Neanderthal’s skull was very robust compared to the flat faced Homo Sapiens.

                We also know the jaw was much stronger than humans because of muscle attachments on the skull and neck. Because there was interbreeding between Neanderthals and our ancestors, some of us even have Neanderthal traits. If you feel on the back of your skull just above your neck you might notice a bony protrusion. I have it and it’s called the occipital bun, it’s an amusing vestige of an older time. Though dentition in Neanderthals was more durable than H. Sapiens, it was still much smaller than that of primitive ancestors.

                The skeleton of Neanderthals appears very hearty when compared to humans. The hips and shoulders were much broader, the rib cage was more barrel shaped, and the limbs were shorter relative to body size. Many scientists feel this morphology contributed to the extinction of Neanderthals. But speculating on the extinction of our close relatives is a debate for another time. As I mentioned Neanderthals and Homo Sapiens did interbreed and we can still find artifacts of them in our DNA.

Denisovans:

Unfortunately there is not a great deal of fossil evidence for the Denisovans. However, we have extracted genetic material from a bone found in Siberia. We know that this species was present in Asia and parts of Oceania.  Amazingly we have found evidence of Denisovan DNA in human beings living in southeast Asia and Australia, indicating that there was interbreeding between Homo Sapiens and Denisovans.

Homo Sapiens:

Last on your list is yours truly: modern day people. Archaic humans, or merely Homo Sapiens first appeared 300,000 years ago, and hopefully we keep going for at least a little longer. Early people were morphologically human but didn’t display the same advanced civilizational and cultural traits we do today, or at least there isn’t evidence for it. H. Sapiens are immediately noticeable by their flat face, large cranial cavity, and gracile features. We are much less robust than any other ape or hominid. The brow ridge, which is so clearly obvious on other hominids, is much reduced in humans. The skull is more rounded with a high dome. Our mouths can’t even fit all our teeth. Teeth that are very specialized to our modern diet. Our foramen magnum is placed very anteriorly, and we stand completely upright.

                We have long graceful legs excellent for running, walking, and long migration. Our skeletons are less durable than those of other apes and we certainly lack the same strength. Our hands have straight and slender fingers, ideal for very delicate and precise work.

                Culturally we are without compare; from blues to Beethoven and cuneiform to comic books, the human animal is capable of extreme creativity. We are not shaped by our environment, instead it is shaped by us. We have touched the heavens and separated the strands that make up the fabric of reality. Unfortunately we are also given to the greatest cruelty, greed, and folly. We are capable of wiping ourselves from the planet, as we have done with hundreds of other species.

                I hope this look at the past has clarified the origin of our species. It is important to understand where we come from if we want to know where we’re going.

Discoveries:

                I would just like to mention those who discovered the species listed, as well as when and where they were discovered. These explorers contributed incredible information in the search for human origin.

Australopithecus Afarenis: Discovered in 1974 by Donald Johanson in Hadar, Ethiopia.

Homo Habilis: Discovered in 1960 by Mary and Louis Leakey in Olduvai Gorge, Tanzania.

Homo Erectus: Discovered in 1891 by Eugen Debois in Indonesia.

Homo Heidelbergensis: Discovered in 1908 in Heidelberg, Germany and described by Otto Schoentensack.

Homo Neanderthalis: Discovered in 1829 by a collection of scientists and was named by William King.

Homo Denisova: Discovered in 2010 in Siberia by Michael Shunkov and his team.

Homo Sapiens: First discovered by the second Homo Sapiens 300,000 years ago, she was known by her friends a Grog. We were first described by Carl Linnaeus in 1758.