Chapter 8:Apes, Hominins, and Human: Morphology, Molecules, and Fossils

Chapter 8: Apes, Hominins, and Human: Morphology, Molecules, and Fossils
The topics covered in chapter 8 are: the relationship between living Hominoids and their classification, when did the evolutionary lineages between apes and humans split from one another, and anatomical characteristics of human ancestors shared by both the apes and humans.
There have two prominent ways of classifying Hominoids, one being morphology and the second being molecular. However, different results have emerged from both methods, and our current view of Hominoid relationships is derived from molecular systematics.

Morphology and Molecules: A History of Conflict

Systematics of modern Hominoids is concerned with “which species are evolutionary more closely related to which other species, and how this relationship should be reflected in both species classification” (pp196). The conflicts that arose were basically broken down into two groups: who, of the modern apes, was more closely related to humans and when did the split between human apes occur.

Morphological Interpretations
The question of which of the modern apes was more closely related to humans was not of much concern. It was generally thought that all apes were equally distant relatives of humans. Much of the debate was contingent on different fossil specimens. The Dryopithecus, found in southwest France in 1856, was thought to be ancestral to modern apes such as the gorilla, chimpanzee, and possibly the orangutan. A second fossil was the Palaeosimia, was thought to represent an ancestral form of the orangutan and a third fossil, the Sivapithecus, was thought to represent the origins of the human lineage. These fossils were in turn interpreted to suggest that the human lineage and the ape lineage spilt apart very deep in the past, some researchers suggested 15-20 million years ago and others suggested 30 million years ago.
By the 1960’s there was strong support for two points. The first being fossil apes were ancestral to modern apes. The second the split between the human and ape lineages occurred very deep in the past. These perspectives reinforced the conclusion that the apes were most closely relate to one another and all equally distant relatives of humans.

Molecular Studies
Molecular studies utilize “genetic distances to identify times of divergence between the ape and human lineages” (pp 199). The results of such studies have posited that the time of divergence between apes and humans was, much more recent that the morphologists thought, around 5 million years but not greater than 6 million years.
In contrast to the morphological studies it was also thought that rather than the apes all being closely related to one another and equally distant from humans that the gorilla split from the hominoid lineage first, and then the chimpanzee and human, this in turn suggest that the chimpanzee was more closely related to humans.

What were some of the issues with both morphological and molecular methods and how (if at all were they corrected/explained)? Do you believe that either methods still poses unexplained issues?

A shift in Interpretation: Morphology
With the evidence and interpretations of the molecular techniques, morphological interpretation began to shift closer to the genetic interpretation. This shift occurred for many reasons, one being the importance of molecular data, and the second being new insight into morphological interpretations derived from new data. This new data came in the form of new fossils, cladistics, and re-examining previous data sets.
New fossils came in the form of more complete specimens Sivapithecus, which had possessed more complete facial and cranial regions. These new discoveries made it possible to discern a connection between Sivapithecus, an early hominin, and Ramapithicus, ancestral to the orangutan. This led the morphologists to put the split between apes and hominids closer to 5 million years, like the molecular biologists.
The use of cladistic analyses allowed morphologists to examine attributes and determine relationships between humans and apes. This led them to change their previous view of the relationship between humans and apes.
This in turn, prompted analysis to previous data sets and characteristics previously thought to primitive, such as cranial and dental traits, are in fact derived.

Nature of the Hominin Ancestor
Based on analyses and fossils it is possible to determine what the first hominin may have looked like, when they appeared, and where they appeared.
The close relationship between African apes and humans suggests that the earliest hominin may have inhabited a climate much like the African apes presently do, or in other words be “broadly equatorial” (pp 208). Molecular and fossil evidence suggests that the ancestor should have appeared at some point in the later Miocene, or 4-10 million years, but probably much closer to 5-7 million years.
The appearance of the last common ancestor of apes and humans is probably generalized, but also more chimplike. The size would most likely fall between the gibbon and the chimpanzee. The ancestor would have been primarily arboreal, but incorporated bipedalism into posture and movement. It would also have lacked the skeletal features associated with knuckle walking and would possess large cheek teeth covered with a thick coat of enamel.
However, there is no consensus. For example, Pilbeam suggests that because humans and chimps are so closely related that the ancestral form would posses structures or knuckle walking, and thin enamel. However, the hominin lineage lost these features due to bipedal locomotion and a change in diet.

Classification of Hominoids
There are two issues that affect the classification of hominoids. The first is that traditional/ Linnaean classification reflected the anatomists’ interpretations, which tended to reflect great differences between the apes and humans. This method does not reflect genealogy, which is now the accepted method by researchers. The second is that there is a poor fit of genetic differentiation and evolutionary change at the phenotypic level, meaning that it is difficult to tie genetic markers, which establish lineages, with physical characteristics.

Evolution of Catarrhines: the Context of Hominin Origin
Some General Patterns
There are three general patterns that pertain to Catarrhine evolution. The first is: the geographic distribution of fossil Catarrhines does not coincide with areas where Catarrhines are most abundant today. The second is old world monkeys are more numerous and abundant than apes. The third is that early apes were not primitive versions of modern apes (pp213).

Early Anthropoids
Algeripithecus minutus is a 50 million year old species that may be, however it is uncertain, the earliest known anthropoid. It exhibits cranial characteristics of an anthropoid, but other than that it is rather primitive. However, the Catopithecus browni is the earliest undisputed specimen, about 37 million years old, and it is defined as an anthropoid based on cranial and dental characteristics.

The Earliest Hominoids
Hominoid fossils that date to the Miocene are spread throughout Africa and Eurasia. However, the earliest species known is Preconsul (roughly 22 million years old) were discovered in Africa, indicating that Africa is the most likely location for the clade to have originated. Proconsul is thought to represent the earliest hominoid based on cranial and dental features (large relative brain size and increased surface area of molars/broadening of incisors). However, Preconsul displays a number of ape and monkey traits, such as no tail but monkeylike locomotion.
Preconsul is the earliest known hominoid species and is thought to represent the most basal of the primitive apes. Taking this into consideration:
An interesting note is that Preconsul possessed an opposable thumb, which both apes and monkeys lack. In this situation would the thumb represent a primitive trait retained by humans and not by apes and monkeys, or would it represent a primitive trait that humans re-acquired?

Later Hominoids
Around 17 million years ago hominoid fossils begin to appear on continents other than Africa, such as Asia and Europe. However, the forms that appear outside of Africa possess more modern features, but can still be reasonably linked to the apes.
The best known example of these hominoids is the Sivapithecus, possessing orangutan like characteristics.

Where are the Monkeys
Attention has not been focused on the Monkeys because they were not abundant in the fossil record, an indication that the ratio of monkeys to apes has reversed in present times.

Three approaches to Hominin Origins
The first is genetics suggesting that African apes and Humans are closely related. The second is phenotypes of humans and other apes clearly indicate that humans are great apes. The third is that the fossil record and the climatic context show that hominoids evolved during the Miocene primarily in Africa and then spread across Europe and Asia.