1. Biological Evolution of Primates


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Use the diagram below to answer the question: Where did Humans come from?

Humans are Primates

Humans belong to the order ‘PRIMATES’, which is a group that includes prosimians, monkeys and apes. 


Below: Primates - What is a Primate (Odyssey Earth) 5:07. 
Below: Your Place in the Primate Family Tree (EONS), 12:25.
Primates, Hominids, Hominins

Characteristics of Primates

General characteristics of order Primates

Mammals in the order Primate have the following characteristics:

Structural:

Physiological:

Behavioural:

Modes of Primate Locomotion

Most primates are adapted to an arboreal (tree-living) niche. But some have adapted to move on the ground.

Locomotion

Arboreal (tree-dwelling)

Ground

Hominids and Hominins

HOMININS: all the species belonging to the human lineage. This includes modern humans, extinct human species, and all our immediate ancestors (including members of the genera Homo, Australopithecus, Paranthropus, and Ardipithecus

HOMINIDS: the group of all modern and extinct Great Apes (e.g. modern humans, chimpanzees, gorillas, and orangutans, plus all their immediate ancestors).

Bipedalism is the main trait that separates hominins from all other hominids.

Hominins include living humans, our ancestors (pre-humans) and the bipedal apes with whom we share our evolutionary history. 

About 6-7 mya, the ancestor of living humans and chimpanzees diverged into two geographically isolated populations. One of those groups eventually evolved into us, the other evolved into chimps. Thus, we and the living chimps share a common evolutionary ancestor who lived about 6-7 mya in Africa. This is demonstrated by the fact that we share about 99% of our DNA. 

The most defining characteristics of hominins is our bipedal (two footed) locomotion and upright posture. It is a form of locomotion found 6-7 Mya. The earliest hominins also show changes in tooth form that mark a change in diet and social organisation.

You must be familiar with the following species involved in human phylogeny:

Last Common Ancestor

Human and African Apes shared a common ancestor - one that lived in the forest, moved in trees swinging from arms and walking on all fours.

At some point one of the ancestors took the critical step to becoming a modern human - Walking on two legs, making the change from quadrupedal locomotion to bipedal locomotion.

This change led to profound anatomical changes especially in the limbs, pelvis and skull and marked the separation from apes.

Last Common Ancestor

Overview of Trends in Hominin Evolution

Humans are habitually bipedal. It is likely that the development of bipedalism around 4-5 mya is the main trait that separates hominins from all other hominids, and was the driving force of our biological and cultural evolution. 

Unlike biological evolution, which is driven by genetic changes, cultural evolution is driven by the transmission of knowledge, behaviours, and technologies from one generation to the next through learning and social interactions.

Zoom in on parts of the diagram below, which shows the trends in hominin evolution over time. Only the five species representative of the general trends are shown here. 

The hominin fossil record shows clear evolutionary trends towards bipedalism, increased brain size, increased height, and increased technical ability. 

Biological Evolution - Trends seen in skull fossils:

Biological Evolution - Trends in brain volume, height and dentition.

Biological Evolution - Trends in other skeletal features: pelvis, foot, foramen magnum, spine and valgus angle.

Cultural Evolution - Trends seen in stone tool technologies

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Which species are Hominids and which are Hominins?

Australopithecus africanus, Gorilla, Homo ergaster, Chimpanzee, Homo sapiens


Selection for Bipedalism

One of the possible theory for the transition to bipedalism has centered on drastic environmental changes that swept Africa more than five million years ago. Africa (and the Earth) had become significantly cooler and drier. 

As it did, grasslands in sub-Sahara Africa expanded and rain forests contracted. This means that the landscape turned from near-continuous forest into an open grassland with smaller clumps of trees with no continuous forest. 

Due to these environmental changes, different selection pressures acted on the common ancesters of hominids and chimpanzees. 

According to the hypothesis, at least one type of these primates responded to the environmental crisis by venturing more and more into the open grasslands, looking for food, but retreating to nearby trees to escape predators and sleep at night. 

Advantages of Bipedalism

Around 5-6 million years ago some primates began standing up and walking on two legs. Their success presumably improved their chances of surviving and passing on genes favoring this unusual stance and gait, leading eventually to bipedal hominids.

Disadvantages of Bipedalism

How analysis of fossils provided evidence of how Ardipithecus ramadus transitioned into bipedalism in a changing environment.
Comparing the locomotion and skeletal features of quadrapedal chimpanzees, intermediate Australopithecus afarensis, and habitual bipedal modern human. No sound. 
Selection Pressures for Bipedalism
Summary: Advantages and Disadvantages of Bipedalism