Chapter 9: Searching for the First Hominins

Chapter 9: Searching For The First Hominins

Genetic evidence suggests that the Hominin clade arose 5-7 million years ago (pp 229). This chapter deals with attempts to identify the earliest hominins.

The Moving History of the Missing Link
The missing link, transitional species between apes and humans, has changed throughout history. Initially it was thought to be Homo neanderthalensis, but we now know that they were relative late comers in history. Next Pithecanthropus erectus was thought to be a possible candidate for the “missing link”; it exhibited many apelike and humanlike characteristics. The point here is that since the idea arose of missing links that the dates have been pushed back considerably from roughly 150,000 years ago (Home neanderthalensis) to between 6 and 7 million years ago, with the discovery of Sahelanthropus tchadensis. This specimen is congruent with what genetic evidence is telling us, and furthermore it was found in Africa, which is also parsimonious with what most paleoanthropologists believe.

Major Sites of Early Hominin Fossils: South Africa, East Africa, and West Africa
All of the early Hominin sites are located in Africa, such a Laetoli and Hadar. Research initially took place in South Africa and sites such as Sterkfontein were discovered. It was conducted by researchers such a Raymond Dart and much of the research was conducted in cave sites and lime quarries. In South Africa such specimens as Australopithecines, noting there is two separate species, a robust form and a gracile form. Whatever the case this clearly suggests that two hominids lived at the same time and thrived in South Africa.
Attention then shifted to East Africa and was primarily conducted by the Leakys. Sites that were identified here consisted of Olduvai and Lake Turkana. The specimens that were discovered here consisted of Homo habilis, Australopithecus boisei, Homo erectus, Homo ergaster, and Australopithecus aethiopicus.
Until recently fossils were only discovered in South and East Africa. However, important discoveries have been made, for example what is possibly the oldest known hominin, Sahelanthropus tchadensis, was discovered in West Africa.
All of this evidence suggests that early hominins evolved in Africa, and mot scholars agree that they didn’t spread out until roughly 2 million years ago, when Homo erectus decided to take a nice long stroll north.

The First Hominins: Current Contenders –Orrorin, Ardipithecus, and Sahelanthropus
It has been suggested that a good contender for the earliest hominin should be 5-8 million years old and located in Africa. There are three reasonable contenders.
Orrorin: dates about 6 million years old, shows evidence for bipedalism, but exhibits apelike dentition. It was discovered in Kenya.
Ardipithecus: was discovered in 4.4 million year old contexts in Ethiopia, However, earlier specimens have been dated to about 5 million years ago. It also exhibits evidence for bipedalism, however retains apelike dentition.
The last contender is Sahelanthropus: discovered in Chad and is estimated to be 6-7 million years old. It has a hominin like facial structure, reduced canines and thick enamel. It is also possible that it was capable (at least to a certain extent) of upright locomotion. However the remainder of its features are apelike, such as a small rounded cranium and brain, much like a chimpanzees.
What all of this suggests is that Hominins were incredibly diverse and that many co-existed with one another.

Bipedalism
Bipedalism and Human Evolution
There are four characteristics that are important to the human condition, bipedalism, increased brain size, change in dental structure, and the development of culture. However, these characteristics developed at different rates. A key marker of early hominins was the ability to perform bipedal locomotion. Therefore, it is important to understand bipedalism in order to understand hominin evolution (pp240).
Bipedalism is interesting because of the form that it takes is unique to hominins. However, it is also tied to the assumption that bipedalism was essential for the development of tool because it freed the hands. But we have discussed other species that make use of tools that are not capable of bipedal locomotion to the extent that hominins were, such as tool using finches and chimpanzees.

Biomechanics of Bipedalism
There are two characteristics that distinguish hominin bipedalism from chimpanzee bipedalism. The first is that chimpanzees are unable to extend their knee joints. The second is that in bipedal locomotion the center of gravity must shift to the supporting leg and in chimpanzees this is not possible, due to femur structure among other things (pp242-243).
There are a number of adaptations that must occur for bipedalism to be possible.
-curved lower spine
-shorter broader pelvis and angled femur
-lengthened lower limbs and enlarged joint surfaces
-extendible knee joint
-a platform foot, which aligns the big toe with the other toes
-a movement of the foramen magnum toward the center of the basicranium

Ecological Context of the Origin of Bipedalism
It is important to understand that bipedalism was a shift from vertical clinging and leaping, to quadrupedalism, to bipedalism. The environmental context would have to be one that made it advantageous to walk upright and to give up brachiation.

Development of ideas on the Origin of Bipedalism
There are various ideas about why bipedalism developed: it allows for greater stamina, improves predator avoidance, increases thermoregulatory efficiency, warning, a shift in diet/ eating habits, the ability to carry things, and aggressive/impressive displays.

Energetics of Bipedalism: possible implication in its Origin
The development of bipedalism may be linked to changing environments and the availability of resources. Bipedalism is more energy efficient; it allows the hominin to make the most efficient use of their food resources. This type of environmental pressure may explain why hominins developed bipedalism initially. It also allows for a greater distance to be covered by the hominins, which would be necessary if resources were scarce.

Thermoregulation and Bipedalism
Bipedalism reduces the surface area of the body consistently exposed to the sun. This may also be linked to the loss of hair. In an environment where the sun is always beating down it ultimately is advantageous to develop adaptations that aid in regulating body heat.

Time and Energy: the Ecology of a Bipedal Hominin
No adaptation is perfect so in becomes increasingly important to evaluate the cost and benefits of different forms of locomotion. This allows the researcher to understand under which types of conditions bipedalism may be an advantageous adaptation. For example being well adapted for life on the ground makes one less suited for life in the trees.
The major issue that is addressed is not that bipedalism is a uniformly advantageous adaptation, but in what circumstances do the advantages outweigh the disadvantages. This makes it apparent that hominins were at least spending more than half of their time on the ground.