CHAPTER 5: THE FIRST HOMININS

 

CHAPTER OVERVIEW

This chapter introduces students to the development of the hominin line. It discusses the emergence of the australopithecines—including the evidence we have for their anatomical features, lifeways, and relationship to early Homo—as well as the earliest stone tools.

 

CHAPTER OBJECTIVES

1. Know the four “key human attributes” identified by Kottak and be able to discuss their significance for hominin evolution.

 

1. Know the chronology of hominin evolution.

3.     Be able to identify the oldest known bipeds, their approximate dates of existence, and their connection to australopithecines and to Homo.

4.     Be able to identity and distinguish among the six species of Australopithecus discussed in the chapter. Know the differences between gracile and robust australopithecines, as well as the explanations posited for their evolutionary relationship and their connection to early Homo.

5.     Understand what distinguishes Homo from the australopithecines.

6.     Know the sequence for the earliest stone tools. What kind of tools were made?  When did they first appear?  Who made them?

 

CHAPTER OUTLINE

I.  What Makes Us Human?
A. Bipedalism
1.  The most widely accepted theories of bipedalism stress the advantages it provided in a habitat increasingly dominated by dry, savanna-like conditions.
2.  Bipedalism provides the ability to see over long grass for food and predators and to carry items back to a home base.
3.  Bipedalism is more energy efficient and therefore advantageous in a grassland, where resources are more dispersed than in forests.
4.  Bipedalism exposes less body surface area to solar radiation, which facilitates cooling and reduces moisture loss.
B. Brains, Skulls, and Childhood Dependency

1. Compared with contemporary humans, early hominins had very small brains. Brain size has increased during hominin evolution, especially with the advent of the genus Homo.

2.   Natural selection has struck a balance between the structural demands of upright posture and the tendency toward increased brain size – the birth of immature and dependent children whose brains and skulls grow dramatically for several years after birth.
C. Tools

1. Early hominins likely shared the ability to use and manufacture tools as a homology with the apes.
2. Upright bipedalism would have permitted the use and carriage of tools and weapons against predators in an open grassland habitat.

D. Teeth

1. As they adapted to a savannah diet of fibrous, gritty vegetation, it was advantageous for early hominins to have large back teeth and thick tooth enamel.
2. The rotary motion associated with chewing this vegetation also favored reduction of the canines and first premolars (bicuspids).

II.  Chronology of Hominin Evolution

1. The term hominin is used to designate the human line after its split from ancestral chimps. Hominid refers to the taxonomic family that includes humans and the African apes and their immediate ancestors.
2. Although the first hominins appeared late in the Miocene epoch, for the study of hominin evolution, the Pliocene (5 to 2 m.y.a.), Pleistocene (2 m.y.a. to 10,000 B.P.) and Recent (10,000 B.P. to the present) epochs are the most important.

III.  The Earliest Hominins
A.  Ardipithecus is generally considered ancestral to early Australopithecus and, ultimately, to Homo.
B.   Ardipithecus kadabba lived between 5.8 and 5.5 m.y.a., while Ardipithecus ramidus dates to 4.4 m.y.a.
C.  Ardipithecus is generally recognized as the earliest known hominin (although the Toumai and Orrorin finds may represent even older hominins).

IV.  The Varied Australopithecines
A.  There were at least six Australopithecus species:  A. anamensis (4.2 to 3.9 m.y.a.), A. afarensis (3.8? to 3.0 m.y.a.), A. africanus (3.0? to 2.0? m.y.a.), A. garhi (2.5 m.y.a.), A. robustus (2.0? to 1.0? m.y.a.), and A. boisei (2.6? to 1.0 m.y.a.).
B.  Australopithecus anamensis

1. A. anamensis is a bipedal hominin from northern Kenya, whose fossil remains were first discovered by Maeve Leakey and Alan Walker in 1995.
2. Fossils from two sites date to 4.2 m.y.a. (Kanapoi) and to 3.9 m.y.a. (Allia Bay).

3.  A. anamensis may be ancestral to A. afarensis, which is usually considered ancestral to all the later australopithecines as well as to Homo.
C.  Australopithecus afarensis

1. Earliest definite A. afarensis remains are dated at 3.8 m.y.a. and (with Ardipithecus and A. anamensis) strongly support a very recent (8 m.y.a. at the most) divergence from common ancestry with apes, because of the clearly apelike features found in all three species.
2. Like apes, and unlike modern humans, A. afarensis had sharp canine teeth that projected beyond the other teeth, and the lower premolar was pointed and projecting to sharpen the upper canine. Fossils of molars and jaws indicate the beginnings of adaptation to a coarse (seeds, lots of sand) savanna diet.
3. Cranial remains show that afarensis was still remarkably “apelike” is some respects. The brain capacity was only slightly larger than a modern chimpanzee’s, and the jaw and overall body size indicate considerable sexual dimorphism.
4. Postcranial remains, particularly the pelvis, leg, feet, and spinal entry into the skull, all indicate bipedalism, and are thus clearly hominin.

5.  A. afarensis young probably depended on their parents for a relatively long time, facilitating social learning.
D.  Gracile and Robust Australopithecines
1.   Two australopithecine groups lived in South Africa between 3 and 1 m.y.a.: gracile (A. africanus) and robust (A. robustus).
2.   Some scholars believe that a third, hyperrobust australopithecine group that existed in East Africa was a separate species (A. boisei), while others believe this group was simply a regional variant of A. robustus.
3.   There is debate concerning the relationship between the gracile and robust australopithecines, both of which probably descended from A. afarensis.
a.  One model has africanus and robustus as separate species, whose life spans were at least partly contemporaneous.
b.  Another model has africanus and robustus as sequential, with africanus being ancestral to robustus.
c.  A third model has both groups as part of a single polytypic species, representing opposite extremes of variation within that species.
4.    Changes in the teeth, jaw, face, and skull indicate that the australopithecine diet increasingly focused on coarse, gritty, fibrous savanna vegetation.
5.  Brain size increased only slightly between A. afarensis (430 cm³), A. africanus (490 cm³), and A. robustus (540 cm³). These figures can be compared with an average cranial capacity of 1,350 cm³ in Homo sapiens.
6.  Sexual dimorphism is evident through observing the size of canine teeth, as well as other features, in both A. afarensis and A. africanus.

V.  The Australopithecines and Early Homo
A.  Between 3 and 2 m.y.a. the ancestors of Homo split off and became reproductively isolated from the later australopithecines, such as A. robustus and A. boisei, which coexisted with Homo until around 1.0 m.y.a.
B.  Homo Appears
1.  Contemporaneous (2 m.y.a.) sets of teeth, very different in size, constitute the earliest evidence of a split between the ancestors of Homo (H. habilis) and the later australopithecines, such as A. boisei.
2.  The distinctive early Homo trends are a rapid increase in brain size, increasingly elaborate toolmaking, and an increasing emphasis on hunting and gathering, but there remains considerable debate as to when and in what population these trends led to speciation (from an australopithecine to Homo habilis).
3.  Johanson and White (1979) propose that A. afarensis effectively produced two populations, which became reproductively isolated and evolved into the other australopithecines and into Homo habilis (2 m.y.a. – 1.7 m.y.a.).

VI.  Oldowan Tools
A.  Oldowan core and flake tools dating to about 1.8 m.y.a. were discovered by the Leakeys in 1931 at Olduvai Gorge, Tanzania.
B.   Even older stone implements, dating between 2.5 and 2 m.y.a., have been found in Ethiopia, Congo, and Malawi.
C.  The identity of the earliest stone tool makers has been debated by anthropologists for decades.
D.  A. garhi and Early Stone Tools
1.   In 1999, fossils from a new hominin species, A. garhi, dating to 2.5 m.y.a. were found in Ethiopia, along with the remains of butchered animals.
2.   In 1997, stone tools also dating to 2.5 m.y.a. were found at a nearby site, suggesting that they were manufactured by A. garhi.
3.   Bipedalism, evidence of a long period of infant and childhood dependency, and the association of A. garhi with the earliest stone tools all suggest that australopithecines had some capacity for culture.
4.   Cultural abilities developed exponentially with the appearance and expansion of Homo.
a.   With increasing reliance on hunting, tool making, and other cultural abilities, Homo became the most efficient exploiter of the savanna niche.
b.   The last surviving members of A. boisei may have been forced into ever-more-marginal habitats, eventually becoming extinct.

VII.  Anthropology Today: Another Branch of Early Human Ancestors
A.  Hominin fossils dating between 5.8 and 5.5 m.y.a. recently have been found in Ethiopia.
B.   These fossils, originally assigned to the subspecies Ardipithecus ramidus kadabba, are now believed to belong to a separate species, Ardipithecus kadabba, that was a direct ancestor of the later ramidus.
C.  The scholars who discovered the kadabba fossils have sparked debate by proposing that this and two other early hominin species, Toumai (Sahelanthropus tchadensis) and Orrorin tugenensis (both discussed in Chapter 4), may have been members of the same genus.
D.  The recent discovery of these early hominin species pushes the hominin lineage back some six million years—around the time when molecular calculations suggest that the chimp and human evolutionary lines split.

 

LECTURE TOPICS

1.        Students often think of human evolution in terms of one species leading to the next.  This chapter demonstrates that early in our “lineage” this was not the case.  Discuss the significance of having a number of different hominins living in East Africa at the same time.

2.        Why is tracing the exact line of human evolution so important?  What difference does it make when Homo habilis first appeared?  Present the importance of these distinctions and the implications of alternative theories of social life.

3.        Discuss the relationships between anatomical, social, and cultural evolution among early hominins.  For example, you could explore the links between upright posture, the size of the birth canal, a large brain, the capacity for culture, and social complexity.

4.        Given all of the debates concerning hominin evolution (e.g., the relationships between the various australopithecine species, or between Neandertals and anatomically modern humans), students may wonder why they should believe any of the claims made about early hominins.  Carefully explain the limits of such debates—what is being disputed and (perhaps more important) what is not—as well as their implications.

5.         If any are available to you, fossil casts representing the different stages of hominin evolution are an excellent aid in helping students understand the distinctions between hominin species and anatomical trends in hominin evolution.  For comparison, incorporate skull casts of other animals, particularly primates.

 

SUGGESTED FILMS

The Ape That Took Over the World
2001    50 minutes
This film explores the variety of australopithecines that have been found in East Africa. Particular attention is given to a recent find by Meave Leakey that is just as old as “Lucy.”  A BBC Production.

Apes to Man
1995    53 minutes
This film traces the evolution of primates from the first apes to the rise of modern humans.  Specialists discuss the important shifts that took place during the evolution of humans. A Discovery Channel Production.

Challenging the Human Evolution Model
1994    50 minutes
This film investigates the model of human evolution in which the increase in hominin brain size and the transition to bipedalism are simultaneous.  The researchers presented in this film argue that humans became bipedal before their brain size increased.  A BBC production.

The Evolution of Man
20 minutes
This film presents a 2.5-million-year-old fossil find in South Africa and explores the links these fossils have with modern humans.  Part of the series Evolution: The Evidence for Modern Ideas on Evolution.  From Films for the Humanities and Sciences.

History of the Anthropoid: The Search for the Beginning
46 minutes
In this film, Friedemann Schrenk visits a series of important hominin sites throughout Africa and talks with prominent researchers, including Meave Leakey, Phillip Tobias, Tim White, Berhane Asfaw, and Michel Brunet.  Part of the series The Story of Hominid Evolution.  From Films for the Humanities and Sciences.