Plio-pleistocene hominids in South and East Africa – Australopithecines

Australopithecines play a crucial role in understanding human evolution. These early ancestors, also known as “southern apes,” provide insight into the transition from ape-like ancestors to the genus Homo. This article delves into the various species within the Australopithecus genus, their evolutionary significance, and the characteristics that place them in the hominid category.

Meaning and Location of Australopithecines

The term “Australopithecus” is derived from Latin, meaning “southern apes.” This name reflects the geographical locations where their fossils have been predominantly found. These fossils are primarily discovered in South Africa and East Africa. Some notable sites include Sterkfontein, Taung, and Kromdraai in South Africa, and Olduvai, Laetoli, and Lake Turkana in East Africa.

Time Period and Earliest Varieties

Australopithecines existed around 4 million years ago, before the Pleistocene epoch. The earliest varieties of pre-Australopithecines include:

• Sahelanthropus tchadensis: Dating back 6-7 million years.
• Orrorin: Dated to about 6 million years ago.
• Ardipithecus: Existed between 4.4 and 4.8 million years ago.

These species were bipedal but retained several ape-like features, setting the stage for the evolution of Australopithecus.

Sequence of Appearance

The evolutionary timeline of these species shows a sequence of appearance:

1. Sahelanthropus (6-7 million years ago)
2. Orrorin (6 million years ago)
3. Ardipithecus (4.4-4.8 million years ago)
4. Gracile Australopithecus (afarensis, africanus, garhi) (4.2-2.0 million years ago)
5. Kenyanthropus (3.5 million years ago)
6. Paranthropus (robustus and boisei) (2.6-1.0 million years ago)

Common Characteristic Features

Australopithecines are characterized by their bipedalism, reduced teeth size, especially the canine, and other features that classify them within the hominid category. Despite these human-like traits, they retained several primitive features, linking them closely to their ape ancestors.

Two Major Forms of Australopithecines

Australopithecines can be broadly divided into two forms: Gracile and Robust.

Gracile Australopithecines

The gracile australopithecines are so named because of their lighter body build. Key species in this category include:

• Australopithecus anamensis: This species existed between 4.2 and 3.9 million years ago. Discovered at Kanapoi in northern Kenya by Bryan Patterson in 1965, and later by Meave Leakey in 1994, this species shows a mixture of primitive features in the skull and teeth but has advanced features in the body. The tibia and humerus indicate strong evidence of bipedality.
• Australopithecus afarensis: Known from fossils found in Laetoli (Tanzania) and Hadar (Ethiopia), including the famous “Lucy.” This species existed between 3.9 and 3.0 million years ago. It had an ape-like face with a low forehead, bony ridge over the eyes, and large back teeth, but with human-like teeth and bipedal adaptations.
• Kenyanthropus platyops: Also known as the “flat-faced man of Kenya,” it existed around the same time as Australopithecus afarensis. Tim White in 2003 suggested that this might not be a separate genus but a variant of Australopithecus afarensis.
• Australopithecus africanus: Existing between 3 and 2 million years ago, this species was slightly larger than A. afarensis. Discovered in South Africa, it had a brain size of 420-500 cc and displayed more advanced features like a more parabolic jaw and reduced canine size.
• Australopithecus garhi: Discovered in Bouri, Ethiopia, and dating to 2.5 million years ago, this species had a combination of primitive and advanced traits. Its teeth were notably larger, and its limb proportions were a mix of human-like and ape-like features.

Robust Australopithecines

The robust australopithecines, belonging to the genus Paranthropus, had a heavier body build and appeared later than their gracile counterparts. Key species include:

• Australopithecus aethiopicus: Also known as “Paranthropus aethiopicus” or the “Black Skull” due to the manganese deposits that stained it black. This species, discovered by Alan Walker near West Turkana in Kenya, existed between 2.6 and 2.3 million years ago. It had a small brain size (410 cc), a massive face, and the largest sagittal crest among australopithecines.
• Australopithecus robustus: Found in South Africa, this species existed between 2 and 1.5 million years ago. It had a more robust skull and teeth than A. africanus, a massive flat face, and a brain size of about 530 cc. Bones found at excavation sites suggest they might have been used as digging tools.
• Australopithecus boisei: Also known as “Paranthropus boisei” or “Zinjanthropus boisei,” this is the most robust form of australopithecine. Discovered by Mary Leakey at Olduvai Gorge and Richard Leakey at Lake Turkana, it existed between 1.8 and 1.1 million years ago. It had an extremely massive face and cheek teeth, with some molars up to 2 cm across. Its brain size was around 530 cc.

Phylogenetic Status and Evolutionary Theories

There is no unanimous opinion on the taxonomy of australopithecines, but it is generally agreed that about 5 million years ago, there was a single line of Australopithecines in Africa. This line is believed to have divided into two, or possibly three, branches:

1. One branch became extinct, leading to the robust type.
2. The other branch is believed to have given rise to the genus Homo.

New discoveries, such as A. aethiopicus, A. ramidus, and A. anamnesis, have led to the development of the “three-branch theory” of evolution, which provides a more complex picture of human evolution than previously thought.

Conclusion

Australopithecines are a crucial link in the evolutionary chain leading to modern humans. Their study provides insight into how early hominids adapted to their environments, developed bipedalism, and exhibited varying degrees of primitive and advanced traits. The division between gracile and robust forms, along with ongoing discoveries, continues to shape our understanding of human evolution.

1. Discuss the significance of bipedalism in the evolutionary success of Australopithecines. (250 words)
2. Compare and contrast the morphological differences between gracile and robust australopithecines. (250 words)
3. Explain the phylogenetic status of Australopithecines and its impact on theories of human evolution. (250 words)