5 Introduction to Hominid Evolution

Dr. Shailendra Kumar Mishra

Contents of this unit

1. Introduction

2. Who are hominids

3. Methods of studying hominid evolution

(a) Comparative morphology

(b) Fossil evidence

(c) Dating methods

(d) Molecular techniques

4. Fossil records and key discoveries

(1) Miocene fossils

(2) Late Miocene and early Pliocene fossils

(3) Early hominin fossils

5. Views on hominid evolution

6. Conclusions

7. Summary points

Learning Objectives:

1. To develop a general understanding about Miocene environment and its role in hominid evolution
2. To describe various fossil finds discovered from several places in Africa and other continents To understand links among fossils records and their Phylogenetic importance in hominid evolution

INTRODUCTION

It has remained a long lasting interest of anthropologists to unfold evolutionary history of human beings. Excavations were done across the globe and connections between different fossil finds have been traced using various methods of comparison. Hominids are unique primates adapted for a particular lifestyle. Apart from morphological structures of teeth and bones, the bio-cultural approach provides deep insight into the process of evolution. Comparatively rapid evolution of hominids is easy to understand through a bio-cultural lens (Aillo and Dean, 1990).

Pre-hominin fossil records indicate that journey of hominid evolution began around 7 to 10 million years ago (Jurmain et al., 2011). Past few decades have seen several exciting discoveries in African continent to unravel links between earlier primates and hominid fossils. It is believed that primitive hominids and the earliest forms of human family were evolved and remained confined in Africa. Later, the proliferation of species took place in other continents too (out of Africa). The most complete Eocene anthropoid fossils were recovered from North Africa (Jurmain et al., 2011).

WHO ARE HOMINIDS?

The genera and species which are directly linked to human family or show some affinity to humans are hominids. The term has been loosely used together for monkeys, apes and hominin fossils. In strict sense it has sometimes been used for the members of family Hominidae (Janusch, 1969). At least a dozen of hominid genera have survived during middle and late Miocene i.e. a time of major adaptive radiation known as Miocene adaptive radiation. Miocene has witnessed remarkable geological, environmental and climatic changes. Consequently, thick tropical forests gave way to growth of grasslands with tree clusters and even deserts. Such ecological conditions proved a kick for hominid evolution such as larger brain size, bipedal locomotion, and related post cranial modifications. Responding to geological changes Miocene hominoids moved in Europe and Asian regions giving rise perhaps to family Hominidae, which includes living great apes and humans. Several findings from Africa and Eurasia suggest about the links between Miocene hominids with later hominids yet there remains great deal of uncertainties. However, the anatomical and molecular data indicate that the gorilla, chimpanzee and bonobos are more closely related to human than orangutan. Thus hominid is a blanket term which wraps a large array of primates with several phylogenetic diversifications (Janusch, 1969).

METHODS OF STUDYING HOMINID EVOLUTION:

Palaeoanthropology is a multidisciplinary science which uses broad based techniques to analyze every clue left by early hominins (Lewin, 2005). Several methods have been used by the paleoanthropologists to establish links between fossil evidence recovered from various sites.

(a) Comparative morphology: Skeletal remains such as cranial and postcranial bones and teeth were used to compare structural features of fossil finds. Cranial capacity, height of cranial vault, prominence of brow ridges, degree of facial recession, position of foramen magnum, dental characteristics such as size of teeth (molars and premolars in relation to canines and incisors), number of cusps on molars, dental arcade (‘U’ or ‘V’ shaped) are popularly used parameters to classify and correlate hominids. Ratio between arm and leg length, size and shape of pectoral and pelvic girdles to determine ways of locomotion, abilities to use power and precision grips have been important criteria for comparisons (Stein and Rowe, 2011).

(b) Fossil evidence: In the process of fossilization hard animal parts such bones and teeth are replaced by minerals. These fossils are reliable copies of once living forms. Generally, cranial bones and teeth were recovered as hominid fossil finds. Postcranial bones which are prone to rapid disintegration were scarcely found. Fossil finds are not only helpful in comparing skeletal anatomy but they also help to understand their behavioural aspects such as dietary practices. For instance, hominid track discovered by Mary Leakey in Laetoli in Tanzania details well about bipedal locomotion by hominids about 3.6 million years ago. Dietary habit of Gigantopithecus was established by analyzing phytoliths recovered from the surface of their teeth. It suggests that bamboo was an important ingredient of their diet. Phytoliths are silica deposits which have a distinct shape in a particular plant species (Jurmain et al., 2011).

 (c) Dating methods: It is essential to place fossil finds and sites in proper time frame. To do so various dating methods are employed. These dating methods can be clubbed into two basic types i.e. relative dating and absolute or chronometric dating. Relative dating method tells us that something is older or younger than something else, but exact time period is not established. For example, a fossil found at deeper strata (layer) is older than the fossil found at upper strata; it is based on a relative sequence. This method of dating is called as stratigraphic dating. There remain issues about reliability of the method owing to earth disturbances such as river actions, geological disturbances which may shift the strata or materials. Another relative dating is fluorine analysis, which can only be used to date the remains of bones. Relative dating methods like biostratigraphy and paleomagnetism are comparatively less popular methods. Absolute dating method which is most commonly used for hominid fossils is Potassium-Argon (K-Ar) method. Hominid fossils have Potassium-40 having a half-life of 1.25 million years which changes into Argon-40. Carbon-14 (14C) is another well known method in which radioactive Carbon-14 having half-life 5,730 years is used for age estimation of fossils. Inorganic materials can be dated by a dating technique called thermoluminescence. For estimation of age, archeological materials need to be heated at 5000C to measure its thermoluminescence. It is especially used to estimate antiquity of ceramic pots from recent sites. All different dating techniques both relative and absolute have a degree of error therefore cross-verifications are done by using more than one method. Dating techniques help paleoanthropologists to make interpretation of fossil finds (Lewis, 2005).

(d) Molecular techniques: Greater the morphological differences higher the genetic difference (particularly DNA and proteins) between the species. If degree of difference in protein structure and genetic makeup is vast, it indicates that they have evolved separately for a very long time. Sophistications in the methods of hybridization, rapid sequencing, and recombinant DNA techniques have increased opportunities for studies in molecular anthropology. Molecular techniques are now being used frequently to make sound conclusions regarding hominid evolution. Studies of Vincent Sarich and other molecular scientist had changed the discourse on Ramapithecus, an important fossil in hominid evolution. Molecular studies have established that chimpanzee and gorilla are close relatives of human (Jurmain et al., 2011).

FOSSIL RECORDS AND KEY DISCOVERIES

Fossils of early possible anthropoids were mostly recovered from Africa, Asia and Europe. One of the richest sites that have yielded Late Eocene and Early Oligocene anthropoid fossils is Fayum in Egypt (Lewis, 2005). Geologists have explained that during Middle Miocene, the Afro-Arabian tectonic plate came into contact with the Eurasian plate and a land connection was established between the continents which provided way for movements to several hominoid species between the continents. Collision of plates resulted into several geo-morphological formations such as mountains. Tropical forests were reduced and replaced by woodlands and savanna grasslands. Owing to geo-climatic conditions in Miocene, hominoids underwent extraordinary diversification in Eurasia. It is important to discuss some of the hominoid fossils discovered from different parts of the world (Jurmain et al., 2011).

(1) Miocene fossils:

Dryopithecus: the first representative fossil was discovered from St. Gaudens in 1956, in France. After that a number of fossils were recovered from different parts of Europe. The studies on available cranial and post-cranial structures suggest their affinities with living apes (Ciochon et al., 1990).

Pliopithecus: fossil evidences suggest that they appeared around 16 million years ago. Members of this genus were small in size; almost like gibbons of the present days. They were leaf-eaters, semi-brachiators without a prehensile tail, however, they do not show skeletal adaptations like true brachiators such as gibbons and siamangs (Stein and Rowe, 2011).

Proconsul: fossils were primarily reported from western Kenya and have close resemblances with hominoids. They are not very much like an ape however post-cranially they closely resemble a monkey. But some of the features such as teeth link them with other hominids (Lewis, 2005).

Shivapithecus: this is one of the best known hominid fossils reported mainly from Shivalik hills (foothills of the Himalayas). Their teeth had thick enamels on molars, relatively low cusps on premolars. Smaller canines and well defined sexual dimorphism are other important characteristics of Shivapithecus. There has been great deal of difference among Anthropologists regarding their usual diet. Some Anthropologists believe that Shivapithecus were grass and seed eaters and others believe that they could eat fruits and nuts. Lack of sound postcranial skeletal evidence restricts to develop a clear view on their mode of locomotion. However, available limb, hand and foot bones indicate that they were arboreal quadrupedals with ability of climbing and suspension (Stein and Rowe, 2011).

Some of the popular texts on hominid evolution have preferred to group early hominid forms according to geographical regions (i) African forms (23-14 mya) (ii) European forms (16-11 mya), and (iii) Asian forms (15-5 mya). The important examples of African forms are proconsuls; of European forms are Dryopithecus and Ouranopithecus; and of Asian forms are Shivapithecus (Jurmain et al., 2011).

Miocene hominoid fossils are widely distributed in Africa, Europe and Asia yet they are poorly understood by anthropologists till date. However, they are very much like apes and humans but it is extremely difficult to establish their direct relationship with any of the living forms. Only Shivapithecus shares manly facial features of modern orangutan indicating an evolutionary link (Ciochon et al., 1990).

(2) Late Miocene And Early Pliocene Fossils:

Ardipithecus kadabba: the representative fossils were recovered from middle awash region and the surrounding areas of Ethiopia in 1993. The structure of teeth particularly of canines and premolars were having dental complex which is intermediate between apes and humans (Stein and Rowe, 2011).

Orrorin tugenensis: fossils were discovered from Tugen hills of Western Kenya in the year 2000. Evidence of certain stresses on the surface of femur of the fossil is indicative of bipedalism. Yet, there remains great deal of disagreement among scholars about its erect bipedal gait (Jurmain et al., 2011).

Sahelanthropus Tchadensis: fossils were recovered from toros-monalla in northern Chad during 2001. The fossils are about 7 millions years old. The features of skull are very much similar to Chimpanzee except the size of their canine teeth which is smaller than Chimpanzee. Unfortunately postcranial remains are not present to make conclusions about their locomotion and postcranial anatomy (Su, 2013).

Discovery of these fossils are important in understanding hominid evolution across the globe. It is an evident fact that exact links are difficult to establish in making a sequence of hominid evolution on the basis of existing fossil records. Debates are going on to understand their significance in the journey of hominid evolution.

(3) Early Hominin Fossils:

The most important representatives are Australopithecus and Paranthropus fossils reported from several sites of African continent. Majority of these fossils were discovered from the Olduvai Gorge, Lateoli, afar triangle, Lake Turkana, Tuang sites etc. Some of the representative fossils have been very popular such as ‘Tuang Baby’, ‘Lucy’ and ‘Black Skull’ etc. (Hart and Sussman, 1995; Johanson and wong, 2009). The fossils were divided into two categories on the basis of their robustness i.e. gracile and robust. Australopithecus and paranthropus show small cranial capacity, prognathus face, and thick enamel on molar teeth and postcranial skeleton suggest about their bipedal gait, although might not be identical to homo (Jurmain et al., 2011). These fossils manifest several evolutionary characteristics such as evidence of erect posture and bipedalism, deciduous dentition, tough and fibrous diet etc. Which are important in formulating views on hominid evolution. Most of the views become basis for new hypotheses that are generally modified when new evidence is uncovered or new ways of analysis and interpretation of evidence are developed.

During their work in Olduvai Gorge, Louis Leakey found fragments of certain cranium bones having cranial capacity higher than australopithecus and paranthropus. The fossils were named as Homo Habilis (Handy Man) as they were associated with tools (day, 1986). These fossils were related to plio-pleistocene period. Homo Habilis has average cranial capacity about 631 cc which is 20% greater than australopithecus. The members of this group were early toolmakers in Olduvai. The initial discovery of Olduvai Homo Habilis was supported by the discoveries at Lake Turkana. Characteristics such as dentition and shape of skull vault of Homo Habilis were quite distinct from Australopithecus.

VIEWS ON HOMINID EVOLUTION

Discoveries of several fossil remains primarily from Africa and rest of the globe have advanced our knowledge on hominid evolution. The Anthropologists have tried to links modern human and fossil man to several hominid species unearthed by paleoanthropologists and paleontologists across the globe. The associations with lower primates such as Prosimians have also been traced and established on the basis of available fossil remains. Not only the discovery of fossils but the interpretations are very much important in tracing links among the fossils. The task of interpretation is still not complete. In general, anthropologists have not been able to establish a linear relation among the several fossils. It is our hunch to know what has happened to the populations representing various stages of evolution indicated by these fossils.

Looking at the bigger picture of hominid evolution it can be concluded that an especial environment was essential for hominid evolution to happen and largely such environment was available in African continent. In fact, several genera of hominine were present in Africa and interestingly speciation was frequent among various hominine genera which appeared during the process of evolution. It is hard to conclude whether only environmental conditions had proved as a trigger to the process or other factors were also responsible. A complete record on hominine evolution was never met and therefore gaps will certainly remain.

Within the stages of evolution views of Anthropologists vary to arrange species in a definite phylogenetic order. For phylogenetic position of species of genera Australopithecus and Paranthropus, Anthropologists have no agreement in their views. Some of them believe that Australopithecus Afarensis is closely related to early members of genus homo however others suggest about missing links. Similar differences of opinions have been seen among scholars about evolutionary significance of pre-hominin candidate fossils.

CONCLUSION

Study of hominid evolution have remained as the focus of research not only for paleoanthropologists but for archaeologists and scholar of natural history also. Fossils recovered from Africa and other parts of the globe served as basic evidence to unfold evolutionary history of hominids. Plate Tectonic Theory, Geo-Climatic Changes during Miocene and bio-cultural approaches remained major pillars to build phylogenetic history of hominids (pre-hominins and hominins). There exists a great deal of agreement among scholars that early stages of hominid evolution took place in African continent. Several methods were employed, sophisticated techniques were developed and advanced tools were devised to discover hominid fossils and to explain their evolutionary importance with precision. Hominid fossils yielded from Africa ranges from late Eocene and early Oligocene to late Miocene and early Pliocene. These fossils include representatives of Prosiminans to genus homo. During Miocene hominids were significantly prolific as compared to earlier period and the phenomenon is called Miocene adaptive radiation.

SUMMARY POINTS:

• • Evolution of hominids remained a long lasting interest of anthropologists.
  • Bicultural approach provided deep insights into the process of hominid evolution.
  • Climatic conditions during Miocene such as growth of grasslands in place of thick forests had proved to be a stimulant towards hominid evolution.
  • With the advancements in dating methods, refinements in molecular techniques and growth of interdisciplinary research, evolutionary studies got a fresh momentum in the past few decades.
  • Several discoveries of hominid fossils in recent times particularly in Africa provided clearer picture of hominid evolution.

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