Field Note Collaborations for Week Two

Welcome to your second week of lab. This week you will learn about humans as primates, how we compare to other primates, and what happened as the lineage leading to humans split from the lineage leading to chimpanzees, about 5-7 million years ago. First thing to keep in mind is that humans are primates, this means that biologically we are grouped together with species such as apes and monkeys. All primates are mammals, but not all mammals are primates. We are called homo sapiens, homo signifying our genus, sapiens our species.

So this week we will compare a human skull to the skull of a Gorilla and a Howler monkey to see the differences. We will then look at our first ancestors after the Homo-Pan split, basically the first divergence between humans and the other apes. Throughout this, we will discuss what these changes mean, specifically looking at diet, bipedalism and brain expansion. Looking at our ancestors and our currently living primate cousins informs us what it means to be human: how different are we, in what way are we different, and how did that change happen over time.

Video 2: Primate Characteristics

As we have already learned humans are primates. Our closest living relatives are the African Great Apes, bonobos, chimpanzees, and Gorillas. Other primates include New World Monkeys, this is to say monkeys living on the American continent, and Old World Monkeys, monkeys primarily in Africa and Asia. Another group of primates are the Strepsirhinis, previously called Prosimians. They include lemurs living on the Island of Madagascar and groups of small nocturnal primates living in Africa and Asia. Bush babies and Lorises.

Despite the variation in the Primate order, in size, diet, and social organization, some characteristics characterize primates as compared to other Mammals.

Primate characteristics are :

1. Enhanced vision

2. Reduced sense of smell

3. Opposable thumbs

4. Dietary generalists

5. Extensive parental investment

6. Expanding brain

7. Social complexity

As primates, humans exhibit these traits to varying degree and we will compare a human skull with a Gorilla skull to see how humans are different, the same way any primate species is unique,. We will discuss how this is related to specific human adaptations to bipedalism, dietary changes, and reliance on intelligence and tool use. [stop]

Between 5 and 7 million years ago, humans shared a common ancestor with the living species; chimpanzees and bonobos, Pan Troglodytes and Pan Paniscus. About two million years earlier, humans, chimpanzees, and gorillas (Gorilla species) shared a common ancestor, and around 12-14 million years ago, the Orangutangs, Pongo species, split off. Think of this as a tree, with different branches taking off and going in different directions. Gorillas and chimpanzees, and other living nonhuman primates, are not our ancestors but we shared a common ancestor.

Gorillas are a species of African Great Apes, living primarily in mountainous regions and forested lowland. They live in groups with one male, several females, and the young offspring. Depending upon their specific environment, they eat much leafy greens. Contrary to chimpanzees, they are not known to hunt for meat.

Let us look at a modern human skull and a Gorilla skull. Notice first the shape of the skull: the braincase of the human is much larger, making room for an enlarged brain. The face is protruding on the Gorilla, called prognathism. We will see later how these two features have changed on the human ancestors after the split of the human lineage from the apes.

Next, let us look at the top of the skull: the Gorilla has what is called a sagittal crest going across the top of the skull, as well as a nucheal crest, thich bones, on the back of the skull. So what are these for? They are for attaching muscles that help in chewing, primarily tough foliage. The nucheal crest also helps attach muscles for keeping the head up when walking quadropedal. We see this is non-existent on the human skull. This has to do both with dietary changes (fruit, meat, and cooking) as well as a larger skull relative to body size that allows for a different attachment of muscles. The zygomatic arch, the cheek bone is also much more slender in humans for same reason: muscle attachment.

Notice that this is a female Gorilla skull, a male has an even larger sagittal crest. This is another feature that differentiates humans from other great apes such as gorillas and chimpanzees: we have reduced sexual dimorphism, this is, difference in size and features between males and females. Human males and females are less different than Gorilla females and males.

Over the eyes, we see a brow ridge: the subraorbital torus. We have one too, but very small. It is thought that this is again a feature related to strong chewing muscles, there is a need to strengthen the cranium.

Looking at the teeth, we can see large canine teeth in the gorilla. Humans are an exception among mammals in general and primates in particular, to have reduced canine teeth. Canines can be used for chewing and for displays, it is not uncommon in species where males compete for females to have large canine teeth to use in displays of power. Gorillas use them for defense and display. The reduction in canines may be related to change in human social organization but also to differences in the jaw as prognatism was reduced: this is to say, as we evolved a flatter face.

Opening up the mouth, we see the molars: they are larger and flatter in the Gorilla, again related to the need to grind and chew tough foliage and other greens.

Let us turn around the skulls and look at one of the most telling characteristics: the foramen magnum. The spinal cord attaches here, running from the brain through the spine and thereby connecting the head and body. The foramen magnum on the Gorilla is far back, while the human foramen magnum has moved forward. The reason is bipedalism: to keep the head up and sight forward while walking quadrapidally, the attachment needs to be on the back. But for keeping a straight look on two legs, it needs to be right underneath the center of the skull. Else, we would be looking up rather than forward.]

Video 3: Variation in Primates

The primate lineage is highly diverse, and we will now examine variation in primates.

We will look at dentition, placement of foramen magnum, and eye sockets. First a Howler Monkey, a New World Primate living in Southern America, I have interacted with them in Costa Rica. Notice again the foramen magnum , the large canines, and specialized teeth: sharp incisors for tearing, and sharp cusps on the molars for grinding fruit. These eat leaves and fruit.

The next is the Tarsier, one of the most elusive and mysterious primates. A Tarsier is tiny primate living in Asia,

Indonesian Islands and the Philippines. It weighs only between 50 and 140 grams. In contrast to the other primates we have discussed, they are nocturnal, active at night. They have, as you can see, huge eyes, and are able to turn their heads almost 180 degrees, a bit like an owl. It gives them an immense field of vision which helps them in their hunt for insects. Their name comes from enlarged feet bones, tarsal bones, on at least two feet. Notice how all the other primates we have discussed are diurnal, active at day, and can rotate their eyes in the eye sockets. The tarsier is also different in that it is primarily carnivorous, hunting insects. The Gorilla is primarily herbivorous, the Howler frugivorous and and herbivorous, and Homo sapiens is omnivorous, with the big catch that this species cook food.

Video 4: First Hominids

About six to five million years ago, a new form of creature appeared in Africa. They were hairy and short, but started spending more time walking on two legs. We have two fossils from this crucial time of transition: Orrorin Tugenensis and Sahelanthropus. These were the first hominids, a term standing for “bipedal ape, this means it refers to humans and our ancestors”.

In the 90’s, researchers found evidence of an unknown species in Ethiopia. The findings and analysis were published in 2000. Ardipithecus Ramidus lived around 4.5 million years ago, shortly after the human lineage diverged from the lineage leading to chimpanzees and bonobos. A partial skeleton of a female has been nicknamed Ardi. She is about 120 cm (3 feet) and based on analysis of this skeleton and many more fragmented skeleton parts, it has been argued that this species showed less sexual dimorphism than living Great Apes. Ardi was adapted to both climbing (an opposable toe, a grasping toe, that humans do not have but apes have) and longer stretches of bipedalism than apes (which we see in changes in the pelvis and the skull).

Video 5: Lucy – Australopithecine

Moving ahead in time, we meet a group called the Australopiths, often divided into the two genus

Australopithecine and Paranthropus. As discussed in your textbook, this is a very diverse group of species and there is much discussion regarding how many species the fossils actually represent. What is common for these species is a) clear bipedalism, b) small brain, c) large teeth

The famous Lucy skeleton is an Australopithecines Afarensis. This skull is Lucy’s species. Lucy is about 3.2 million years old, and got her name because the researchers were playing the Beatles song “Lucy in the sky with Diamonds” when they discovered her in 1972, in Afar, Ethiopia.

Notice that this skull of an Australopithecine is slender, much slender than the gorilla and lack the crests. The canines are reduced and the foramen magnum has moved to allow for bipedalism. However, it may not have looked exactly like modern human gait. She was short, only just over one meter, and her brain was about a third of a modern human brain and slightly smaller in absolute size than a Gorilla. Adjusting for body size, her brain was probably not much different than a modern Chimpanzee or Gorilla. What this tell us is that the expansion of the brain seen in modern humans had not started yet.

Notice the post-orbital constriction and how this is less pronounced than the Gorilla but in modern Homo sapiens, it is absent.

Our next skull is the Paranthropus robustus. Notice the crest and much larger molars: they are likely for chewing roots and other hard to chew food. The crest could be even more pronounced, making it likely this is a female. It was found at Swartkranz, South Africa. Later Paranthropus Boesie in East Africa had even larger teeth.

This specimen is from Olduvai Gorge, East Africa. Notice the extremely large teeth. This is an Paranthropus Boesi, and it has taken the robustness of our previous species to the extreme.

Around 2 million years ago, something different happened: the Genus Homo appeared. We will discuss that in the next lab. We have no evidence that any hominid species prior to Homo left Africa. This is to say, Australopithecines stayed in Africa. Likely there was much diversity as we have seen in the skulls presented, and it is not yet absolutely clear which lineage led to the Genus Homo.

Following script was cut from Week 2 Lab/Video. It is to be used as an Optional Video to be accessed from Week 2 Lab.

Until recently, it was thought that the use of tools, in particular the manufacture of stone tools, was reserved to members of the species Homo. However, we now know that Chimpanzees use tools in the wild and can manufacture stone tools if taught by a human. Some evidence also suggest that some Australoptihecines made stone tools: we find stone tools associated with Australopithecine remains at Olduvai George where the first Homo remains were also found – though they have been associated with the Homo habilis remains, it is not clear which species created them. We have also found evidence of cutting marks on animal bones going back before 3 million years ago, more than 600.000 years before the oldest identified remains of a member of the Homo genus. This is an intriguing debate and new findings are just going to deepen our understanding of how our ancestors lived and when the behaviors and traits we consider humans actually evolved.

To recapture, our knowledge of our past and our evolutionary trajectory is based on a) comparison with living primates, especially the great apes who are our closest living relatives, 2) Discovery of bones and other remnants. Dating technique allow us to know when they lived, and examination of their physical features help us understand how they lived and what they looked like, 3) DNA and genetic studies help us further establish when a specific change in our genome occurred or how close we are to living or past relatives