Stephen P. Broker
1. Comparative morphology
Natural relationships among organisms can be fairly well determined by comparing their various structural features. Modern forms of life have been classified into related taxonomic categories (genera, families, etc.) based on similarities in foot structure, teeth, skull shape, method of reproduction, type of locomotion, and a number of other characteristics. Among the modern primates, for example, the prosimians, Old World monkeys, New World monkeys, apes, and man are recognized as distinct groups.
Comparative morphology is used in studying extinct forms of life whose skeletal remains have been recovered. Important similarities suggest relationships among earlier species and link some of them with present species. Scientists can develop histories of evolving species (phylogenies).
Some of the morphological features which help to classify hominids
are:
-
cranial morphology—
height of the cranial vault; cranial capacity; presence of a sagittal crest on top of the skull (for attachment of strong chewing muscles); presence of brow ridges (for muscle attachment); degree of facial recession; position of the foremen ma B um (the opening at the base of the skull through which the spinal cord passes).
-
dental characteristics—
size of the cheek teeth (premolars and molars) in relation to the front teeth (incisors and canines); number of cusps on the molars; shape of the dental arcade (Ushaped, Vshaped, or parabolic.)
-
postcranial characteristics—
the intermembral index (the ratio of the length of the arms to the length of the legs); shape of the pelvis (the entire complex of characteristics that determines the method of locomotion—brachiation, knucklewalking, or bipedalism); manual dexterity (ability to use the power grip (first clenched on an object) and the precision grip (thumb and opposing forefinger— the OK sign.
2. Fossilization
The hominid fossils that have been recovered in Africa, Europe, and Asia over the past 120 years are the products of mineralization. This natural process has occurred by the seeping of minerals into the hard tissues of animals or plants which had been fortuitously covered by sediment, Fossils are faithful copies of onceliving organisms. When an animal skeleton is fossilized, the bone material is completely replaced by minerals.
Until fairly recently, very few hominid fossils had been located, and it could be said that all such fossils collected together would be held by one goodsized table. A number of continuing expeditions for fossils in wellknown sites in the Old World have produced greatly expanded collections that have increased our knowledge of hominid evolutionary history. There remain significant gaps in the fossil record, however. Postcranial remains, for example, fossilized ribs and vertebrae, arm and leg bones, hand and foot bones, and pelvises, are scarce when compared to the number of tooth and skull fragments that have been found. The postcranial bones are smaller and softer and tend to disintegrate (or be scavenged) much more readily than the hard bones of the head.
In terms of a temporal record of hominid evolution, there is a serious gap in our knowledge; we know little or nothing of the period from 8 million to 4 million years ago, No wellpreserved and welldocumented hominid fossils have been found from this four millionyear span of time. As this may well be the period during which apes and hominids diverged from a common parent stock, future discoveries from this age will be particularly important.
Evidence of earlier types of life can also come in the form of imprints or casts. The track left by a crawling worm can be fossilized, as can footprints, such as the wellknown dinosaur tracks of the Connecticut River Valley. The recent announcement by Mary Leakey of 3.6 million year old hominid tracks represents an important new way of learning about hominid structure and functional adaptation.
Some hominid skulls have been found with associated natural endocasts. These formations are the result of material filling the skull and becoming part of the enveloping rock matrix. An example of such an accurate copy of the inside of the skull is the Taungs baby, which lacked the back portion of the skull but did have a well preserved endocast, showing how large the skull was.
3. Absolute and Relative Dating
The age of one fossil with respect to another can be determined by one of the methods of relative dating. A principle of geology is that sedimentary or volcanic formations at an undisturbed site are increasingly older as one goes further down in examining the different layers. The most recent layers of rock in a geological formation are at the top. Consequently, a fossil located in a layer of sediment is of an earlier age than one found in the layer above. It has been found that two sites separated by considerable distances may have some common sequences of rock layers. These comparable layers are assigned the same approximate ages. By considering two or more sites together, an expanded cross section of the earth’s layers is seen. Fairly complete chronological histories of a geological region are thus obtained.
The same types of fossil plants and animals are often found in comparable layers at two sites. If the rock layers at one of the sites are not easily distinguishable, comparisons of the fossils at each site can help determine the correct layer. Such floral and faunal comparisons help to date hominid fossils found in poorly defined layers or in a stratigraphy which is subsequently destroyed. The limestone deposits of South Africa, for example, have been destroyed by quarrying or are not suited for absolute dating techniques. Approximate ages of these deposits have been obtained by making faunal comparisons with the welldated deposits of sites in East Africa.
Absolute dating assigns a specific age to a layer of rock or a fossil. These techniques involve an examination of the levels of radioactivity in rock samples. Organic remains, volcanic material, and sedimentary rock contain unstable isotopes of elements that undergo continual decay. The relative amounts of the radioactive isotopes present in the organism when it was alive, in the lava and ash when they were released from the volcano, and in the sediment when it was laid down are known. Also known are the rates at which the various isotopes decay to form more stable products. Potassium Argon dating is the technique best applied to early hominid and pongid fossils, which are several hundred thousand to more than 10 million years old. An unstable form of Potassium, K40, which is present in volcanic ash, breaks down to form Argon40. The halflife (tt = the amount of time it takes for one half of the unstable atoms present in the original material to decay) of K40 is 1.3 billion years. KAr dates reflect a degree of uncertainty of this process by showing a range of years. Thus, the ER1470 skull described by Richard Leakey was dated at 2.50=0.25 million years before present.
4. Endocranial casts and microwear of teeth
The process of fossilization sometimes produces natural endocranial casts which conform to the inner contours of the skull. Synthetic polymer casts can also be prepared, in a technique developed by Ralph Holloway. Holloway has prepared casts of six South African australopithecine skulls, as well as endocasts for East African australopithecine,
Homo habilis
from Olduvai, and the 1470
Homo habilis
from Koobi Fora. The casts give accurate measurements of cranial capacity, and outlines of blood vessels of the membrane surrounding the brain. The size of the major subdivisions of the brain—socalled gross brain morphology—can be determined.
Holloway concludes that
Australopithecus
had a brain which was essentially human in organization and that its brain was in the same general proportion to the size of its body as is ours. His data would tend to support the idea that a humanlike brain was present in hominids 3 million years ago, even though rapid expansion of the brain took place much more recently.
Within the last year another technique for studying hominids has been developed. Alan Walker is studying the microwear of teeth of living and extinct mammals. He uses fossil and modern teeth to prepare epoxy replicas which are then examined under the scanning electron microscope for microscopic wear patterns. The scratches and pits on teeth provide clues as to the diets of each organism. Grazing animals, which eat grasses, have teeth with a number of microscopic pits or abrasions on their surfaces. Grasses have large quantities of silica crystals in their cells which scratch tooth enamel. Browsing animals feed on the leaves, branches and fruits of trees and bushes. These plant materials have fewer silica crystals in their cells, and a more finely polished tooth surface results. Omnivorous animals, eating meats as well as plant material, scratch their teeth heavily when biting into bone.
By comparing tooth wear for mammals whose dietary habits are known with wear on teeth of extinct hominids, the diets of early hominids can be determined. Walker’s initial studies show that
Homo erectus
was the first omnivorous hominid, and that the australopithecines and
Homo habilis
were herbivorous, principally eating hard fruits. This would require that early hominids fed in forested areas, rather than exploiting open grasslands, as is generally believed.
5. Molecular evolution
Paleontologists assume that the greater the differences in morphology between two species, the more remote in time is the ancestor common to both species. Molecular evolutionists argue that this is equally true for the genetic and protein differences between species. If two types of life are very different in terms of their protein structure and genetic makeup (when comparing the same proteins and genes), then they have evolved separately for a very long time. A number of scientists are now studying evolution at the molecular level. Among them are biochemists Vincent Sarich and Allan Wilson, who are studying blood proteins in a variety of living organisms. Their approach to the study of evolution is advantageous in that they can quantify their date and they can compare forms of life that are extremely different morphologically.
One of the assumptions made by molecular evolutionists is that gene mutations and consequent protein changes occur at a relatively steady rate, much as radioactive isotopes experience constant decay. If this is true, then evolution has a builtin molecular clock, The ticks of the clock are the substitutions in nucleic acid base pairs or the amino acid replacements in proteins. Using the recently perfected methods of hybridization, rapid sequencing, and recombinant DNA work, the nucleic acids and proteins of different organisms can be compared. The number of amino acid differences in the blood albumin of two different species, for example, can be determined, and the duration of evolutionary time needed to achieve those differences can be calculated. By using the minimum number of gene mutations or amino acid substitutions needed to get two different proteins, the time of divergence of the two species is found. Different molecules are found to evolve at different rates. If one is able to determine the correct rate of evolution for a given molecule, this serves as a new indicator of the time of species divergence.
Data collected so far indicate that there are strong molecular similarities among humans, chimpanzees, and gorillas, and that these two great apes are genetically as similar to humans as they are to each other. The time that hominids diverged from the apes is set at approximately 5 million years ago. This is much earlier than the 14 to 8 million year divergence time set by paleontological studies. With the present relegation of
Ramapithecus
to nonhominid status, however, the earliest known hominid is now 3.8 million years old. The molecular clock is now playing a bigger part in evolutionary theory. Some researchers regard it as a sloppy clock, but one worth referring to.