Humans in the Natural World Humans are vertebrates with, among other features:

1. an internal, segmented vertebral column2. an internal, segmented skeleton.3. bilateral symmetry.

Humans are also mammals, with:1. maintenance of constant body temperature.2. birth of live young.3. mammary glands to nurture the young over a prolonged period of growth and development.4. large brain associated with ‘intelligence’5. A dentition made up of several different kinds of

teeth (incisors, canines, premolars and molars) that function together to allow the animal to chew.

Humans in the Natural World

Humans are Primates:1. Basic adaptations to life in the trees (arboreal way of life).2. The retention of the five digit hand and foot.3. Development of acute vision with depth perception.4. Relative lack of an acute sense of smell.

Humans are Higher Primates (with monkeys, apes):1. Social Living2. Daytime activity cycles (diurnal)3. diverse dietary patterns.

Chimpanzees as Humans in fur Chimpanzees as Humans in fur suitssuits

Because of their biological and behavioral similarities to humans, chimps are often used as models of what our earliest ancestors were like.

The major problem with this is that living chimps are the result of a separate evolutionary history as long as ours, and they may have changed as profoundly as we have.

There is not a single fossil bone documenting the evolutionary history of either chimps or gorillas.

Human Origins in Africa

Our closest living relatives, the African apes, the chimpanzee and gorilla, are both native to Africa.

(It should be emphasized, again, however, that there is no fossil evidence documenting the evolutionary history of either of these two primates.)

All of the earliest fossil evidence of our own evolutionary history is limited to Africa, and it is only relatively late in human evolution that our ancestors are found in Eurasia.

There are fossil deposits of this early time in Europe and Asia, and they show no trace of our early ancestors.

What’s in a name?

Although there is much debate about what terms to use when referring to our extinct ancestors, in this class, we will call them hominids, and speak of human evolution as the evolution of the hominids.

Thus, we can say that hominid evolution seems to have begun in Africa, and it is only much later in our evolutionary history that we find signs of our ancestors outside the African continent.

The Time of Human Origins Studies of the genetic similarity of humans, chimps and

gorillas indicate that we last shared a common ancestor with these animals between 5-8 million years ago.

Human evolution can be said to begin at that time. Currently, we have definite fossil evidence earliest from

about 4.4 million years (myr), and recent discoveries now suggest hominids were around at 6 myr.

If the earliest of the genetically derived dates is correct (8 myr), then we have fossils documenting only the last half of human evolution.

If the later date is correct (5 myr), then it would now appear that we have the earliest fossils of the hominid line, and these irtually the entire human fossil record and the earliest fossils should be expected to document our initial beginnings.

Human Origins

An evolutionary diagram of human and African ape relationships, based on various genetic studies

humans chimpanzees gorillas

common ancestry of humans and African apes

5 - 8 myr

10 myr.

Hominid evolution

What is a human?As Profs Liberman and Urban have already noted, modern

humans are unique in having:1. Complex culture with a wide variety of societal

norms in behavior.2. Language and symbolism.2. Biologically:

very large brains in comparison to our body size.

small, non-projecting faces. bipedal locomotion.

The major question here is: when did these distinctive features of modern humans appear in the course of our evolution?

Identifying the Human Lineage

The single most important feature in identifying a fossil as a hominid is bipedalism. This appears to be the earliest major change in our evolution and fossil evidence for erect posture has become the most important defining feature of the hominids.

There are also major changes in the the teeth.Apes possess large, projecting canine

teeth, while hominids have reduced canines.

Evidence for Human Evolution Comparative Anatomy

The examination of the gross anatomical features of humans and apes reveals their close similarity.

Comparative GeneticsComparisons of the genetic materials of the

African apes and humans documents that these creatures are more closely related to each other than to any other living animal.

Fossil Evidence

Fossil Evidence The direct evidence of our extinct

ancestors. Other evidence documents

relationships; only fossil evidence provides data on the biology and adaptation of our actual ancestors.

Because of the nature of the fossilization process, this evidence is often difficult to fully understand and interpret.

The Human Fossil Record Definite fossil evidence documents a beginning more

than 4 million years ago; recent finds of fossils from western Kenya dated to more than 6 million years ago (‘Millennium Man’) suggests a much older origin.

The earliest evidence is of animals possessing some biological traits like those of modern humans, some that resembled the apes, and many unique traits.

Later-in-time hominids look more and more like living people.

Based on anatomical features of the skull, teeth and body size, a number of different genera and species have been proposed.

The Earliest Hominids Found in 1994 in East Africa, these creatures are called

Ardipithecus ramidus. A considerable number of fossil bones of Ardipithecus have

been found ; they are dated to about 4.4. million years ago. The fossil bones are currently being studied, and not much

has been published about these creatures. It is known that they were bipedal, and unlike any later

hominids, and like the apes, they had thin layers of enamel on their back molar chewing teeth.

Most anthropologists believe that Ardipithecus represents an side branch of human evolution that had no descendents.

The Earliest Hominids

millions of years ago

2

3

4

5

6

Homo

AustralopithecusArdipithecus

The Australopithecines First identified by Raymond Dart in 1925.

The fossil was that of a young child from the site of T’aung which is a limestone quarry in the Cape Province of South Africa.

Dart examined the fossil and found several features he believed identified this child as a human ancestor.

He named the fossil Australopithecus africanus (‘the southern ape of Africa’).

A Question to Consider

If you did not know about the fossil evidence, how many of you would have expected our early ancestors to be distinguished first and foremost by increased head size?

This is, after all, the seat of our intelligence and ‘humanness’.

The Australopithecines All are bipedal and therefore hominids. Now known from sites in East, South

and Central Africa. They are dated from about 4 to about 1.5

million years ago. On the basis of differences in features

like size of the teeth, the brain and body, the australopithecines have been divided into more than six species.

The AustralopithecinesAlthough there are at least six recognized species, all

australopithecines possessed in common the following:

1. the anatomy of bipedalism.2. small, non-projecting canines.3. incisors not much bigger than in humans.4. back chewing teeth, the premolars and molars

that were huge.5. very large jaws and jaw muscles.

The Australopithecines

6. australopithecines had small brains:

volumes range from 400-500ml.

By comparison:

modern humans: 1400m.

chimpanzees: 375m.

gorillas: 500ml.

(1000 ml = 1 liter, 1 US quart = 946 ml.)

Australopithecus afarensis

Although not the earliest of the australopithecine species, this is one of the most famous, with many fossils, and other evidence, unearthed from sites in East Africa: Hadar (Ethiopia) and Laetoli (Tanzania).

Dated between 3.7 -2.9 myr. At Laetoli (3.7 myr), in addition to fossil bones, Mary

Leakey found three footprint trails. At Hadar (3.2-2.9myr), many fossils have been found

including the most complete australopithecine known: about 40% of a skeleton called ‘Lucy’.

Australopithecus afarensis ‘Lucy’ is the partial skeleton of an adult

female (without skull) who was about 1.1m tall and 27 kg. (3’ 6” and 59 lbs.). ‘Lucy’ had a completely bipedal skeleton, with minor differences from modern humans, including the possibility that she may have spent some time in the trees.

This is confirmed by the footprints discovered at Laetoli.

Australopithecus afarensis In the size and proportions of the skull,

A.afarensis had a unique combination of biological traits.

The back chewing teeth of A.afarensis are very large, but the canines are human-like.

A.afarensis had a brain size like those of chimpanzees, and there was probably significant body size differences between males and females.

Sexual Dimorphism

Sexual Dimorphism Sexual dimorphism is the difference in body size

between males and females of the same species. In primates, if sexual dimorphism is present, the males are always larger than the females.

There appears to be marked dimorphism in all australopithecine species.

In primates, dimorphism is associated with male dominance; there are no dimorphic primate species with pair bonding, which is the basis for the human family organization.

Sexual dimorphism in modern humans?

What do you think?

Is there a set of differences that might distinguish males and females?

If sexual dimorphism is no longer present in modern humans, what do you think are some of the reasons it disappeared during the course of human evolution?

Evolutionary Relationships Anthropologists have suggested two different

views of australopithecine evolution, and their eventual relationships to later hominids and us.

1. The australopithecines can be placed within a time ordered evolutionary lineage, with the earliest evolving into later forms, and ultimately, into early members of the genus Homo, who will eventually evolve into modern humans.

Australopithecine Evolution: Scheme 1

1

2

3

4

5

millions of years ago origin of hominids

Ardipithecus

XA. anamensis

A.africanus

early Homo

A. aethiopicus

A. robustus

A. boisei

A. afarensis

A. gahri

Evolutionary Relationships II

2. Early in australopithecine evolution, they spread into different environments, and underwent divergence, evolving into a number of different species, each with somewhat different biological features, yet all possessing the ancestral traits. In time, only one species evolved into Homo.

Australopithecine Evolution :Scheme 2

1

2

3

4

5

A.africanus

millions of years ago origin of hominids

ArdipithecusA. anamensis

A. afarensis

early Homo(several species)

X

A. aethiopicus

A. boisei

A. robustus

A. gahri

So, Just what do we know?

The hominid line begins at least four, and now probably six million years ago.

The australopithecines were our early ancestors, with features including:

1. bipedalism.

2. large back teeth, and big chewing muscles.

3. brains sizes similar to those of the apes.

4. some (all?) were sexually dimorphic.

5. It is unclear if they made tools. Most have not been found with tools.

Australopithecine Adaptations and Behavior

Although the number of fossil specimens attributed to Australopithecus is considerable, the nature of the deposits in which they are found limit the reconstructions that are possible.

1. Taphonomic investigations are only partially able to reconstruct the habitats of the various species, and questions remain about their ability to utilize a mosaic of environments, as do chimpanzees.

2. The anthropologist Jane Lancaster has observed that “the australopithecines are all extinct, and so are their behaviors”. How much like living animals and human gatherer/hunters these creatures were remains an open question.

Australopithecine Adaptation

It is unclear whether the various australopithecine species occupied different environments, or whether, like chimpanzees, they were capable to utilizing a variety of seasonally related habitats, including forests, woodlands and open grasslands.

It also uncertain just how widely distributed these creatures were. Did they live throughout Africa?

Outside of Africa?

Australopithecine behavior

Very little is known about specific aspects of australopithecine adaptation and behavior.

Stone tools are found earliest at about 2.5myr ago, but unassociated with any fossils. South African australopithecines are found with stone tools, but it is unclear whether they, or a more advanced hominid, made the tools.

There is some evidence that by about 2.0 myr, early members of our own genus, Homo, had appeared; these may have been the toolmakers.

Australopithecine Behavior II Little specific information is available about what

these creatures ate, and the possible differences in diet between the various kinds of australopithecines.

It is often assumed that those australopithecines with the huge molar teeth, the so-called ‘robust’ australopithecines, were eating a diet of coarse, gritty vegetable foods.

However, the smaller ones, like ‘Lucy’, known as ‘graciles’ also possessed very large teeth for their body size, and both kinds of australopiths had thick enamel on their teeth, and also had very heavy dental wear.

Early Hominid Environment

Reconstructions of Australopithecine Behavior

Investigations of australopithecine behavior have focused on:

1. Were they bipedal chimps or humans in fur suits.

2. Brain size and brain and body size estimates.

3. Patterns of maturation.

4. Dietary reconstructions.

5. Social organization.

6. Tool use.

oldowanpebble

oldowanmaking

bloodymess

zebra

smashbone

scratchmaking

Evolution of the genus Homo

There are fragmentary and difficult to interpret fossils from about 2.4 myr. that may be the first evidence of the appearance of members of our genus, the genus Homo.

By 1.9 myr however, there are clear indications of the presence of the genus Homo from fossil finds in Kenya.

There appears to be several species of early Homo at about 1.8 myr.

Early Homo First documented appearance of members of

our genus is just after 2.0 million years ago. There are fragmentary fossil bones from

slightly earlier, about 2.3-2.4 million years ago, but because of their incomplete nature, they are very difficult to interpret .

The earliest well documented fossils have all been found at sites east of Lake Turkana, in northern Kenya. Somewhat later are fossils from Olduvai Gorge, Tanzania.