Stephen P. Broker
Through the scientific efforts of the past several hundred years, we now know that the earth is an ancient planet, that it has undergone extensive physical change, and that it continues to evolve in appearance. We also know that all forms of life undergo changes in appearance and functional ability over the course of generations. Fossilized remains of earlier forms of life indicate that different species evolve at different rates. The socalled “living fossils,” such as the Ginkgo, the cycads, the dragonfly, the shark, and the coelocanth, have remained largely unchanged for millions of years. They contrast with such organisms as the horse and the elephant, whose evolutionary histories of 40+ million years show major changes. Horselike forms, for example, underwent changes from foot high, fourtoed mammals to the fiveto six-foothigh, onetoed forms of today.
Species are mutable. It is also true that many earlier forms of life are extinct—in excess of 90% of all life forms ever to have inhabited the earth. Some of these forms have become “extinct” in the sense that they have been transformed through genetic mutation or hybridization into new and different forms. Many types of life have been evolutionary deadends, however—evolutionary experiments which, though they may have been welladapted for long periods of time, did not enjoy the luxury of descent to the present. (It should be realized that despite use of the term “evolutionary deadend,” with its unfavorable connotation, no suggestion of inferiority or of reduced significance for lineages that have died out is intended. The one constant in the biological history of the earth is change.)
Despite protestations to the contrary, man too has experienced change in form and function over a long period of time. We are animals, more specifically mammals, whose present exalted state represents a unique response of reproducible genetic material to environmental forces. That unique feature about us which is most prominent and most deserving of (our own) awe is our large and complex brain, an organ which is capable of the most sophisticated of functions. Green algae, grasshoppers, and gibbons have their own unique features. Such a realization in no way diminishes appreciation for the marvels of the human species; it merely heightens appreciation.
A primate, man bears many obvious similarities to the great apes, monkeys and prosimians, the other types of primates. Chemical studies of blood proteins have shown that the great apes—chimpanzees and gorillas in particular—are more closely allied in genetic makeup to man than they are to the other primates. The implication is that they are more closely related to us phylogenetically. The developing discipline of molecular evolution suggests a divergence of hominids away from pongids as recently as five million years ago. The earliest known anthropoids are 40 million year old (Eocene) forms, found in 1978 in Burma by Russell Ciochon and Donald Savage.
Amphipithecus
and
Pondaungia
are believed to be at or near the point of divergence of apes and monkeys. A possible 40millionyear separation in development for apes and monkeys is to be compared with a possible 5 million year separation of the apes and hominids.
As has been stated, the early hominids, organisms directly or closely related to our own species and including
Homo
, have been recovered from once tropical savanna and open grassland regions of Africa, Asia, and Europe. Most individual organisms of past eons did not leave fossilized evidence of their existence; rather, they underwent total decomposition, the result of weathering, scavenging, decaying. This is particularly true in tropical areas of the world, and it is particularly true of the hominids. Hominid remains are fragmentary and incomplete. Despite the great increase in recent years of the fossil evidence for hominid evolution, when one considers the long time during which hominids have evolved, the large numbers of generations of individuals that span the past 5 to 10 million years, and the extreme unlikelihood that the complete fossil record exists or will be recovered, it is safe to say that direct linear descent of man from his precursors will never be established. What we do have available to us is best described as evidence of occasional moments in hominid evolutionary history, a fragmented, threedimensional jigsaw puzzle with a fourth, temporal dimension added.
We need not be committed to total disappointment in trying to satisfy our curiosity about our origins. Examination of the fossil evidence at hand can reveal much about lines of development and evolutionary trends undergone over time by members of the Family Hominidae. Major trends in hominid evolution include refinement of bipedal locomotion (probably the most significant evolutionary development), increase in brain size, and refinement of stereoscopic vision with a concurrent reduction in the importance of the sense of smell. Shortening and flattening of the face is related to the abovementioned changes.
In the 120 years since Charles Darwin delineated his theories of evolution in
On the Origin of Species
, all aspects of Darwinian evolution have come under attack. Darwin was familiar with a number of extinct fossil forms, having collected a variety of fossil remains while serving as a naturalist on board the H.M.S. Beagle in the 1830s. The fossil evidence for human evolution was sparse in the second half of the nineteenth century, however, and it was of little help to Darwin in his formulation of theories of man’s descent. The principles of Mendelian genetics have served to reinforce Darwinian theory, while recent genetic studies using techniques of DNA recombination, hybridization, and gene sequencing have revealed a more complex process of protein synthesis, and raised new questions about the importance of natural selection to the evolutionary process. Nevertheless, Darwin’s basic formulations are still generally regarded as valid.
Species adaptation through natural selection, according to Darwin, is the principal vehicle for evolution. Darwin recognized that all living things are theoretically capable of rapid and enormous increase in numbers—geometric increase. Most populations of organisms, however, experience arithmetric increase, or tend to remain fairly constant in numbers over time. Darwin concluded that forms of life seldom if ever enjoy optimum conditions for growth and reproduction, that they are all faced with a struggle for survival. At the same time, species are made up of collections of individuals exhibiting a variety of morphological and functional characteristics. Adaptations can be of greater or lesser advantage to individual organisms. Ultimately, some organisms are more successfully adapted to their surroundings than others. (These adaptations, it should be emphasized, arise randomly and passively.) Environments eventually change, and new advantages may be bestowed on previously insignificant adaptations. Over a period of time, those individuals which are best adapted will tend to survive and pass on their traits through reproduction.
Development of new species is the fundamental process of evolution. This process, called speciation, is believed to occur in one of the following fashions. A population of individuals of the same species inhabits a distinct, perhaps widespread, geographical area. Individuals at one extremity of the population range may be somewhat different from individuals at the other extremity of the range. There is a continuity of characteristics across the range, however, and although individuals at one extreme may not come into contact with or mate with individuals at the other extreme, they remain capable of successful mating and production of viable, fertile young. They are also reproductively isolated from other species. These are the criteria for determining if two organisms are of the same species.
It is at this point that theories differ as to how speciation takes place. One school of thought says that genetic drift—the gradual accumulation of neutral, nonadaptive gene mutations in stable populationsis the principal driving force of evolution. Darwinian evolutionists, on the other hand, maintain that natural selection by survival of favorable adaptations is the prime mover. They disagree among themselves as to the method of speciation. The theory of sympatry holds that two species can develop from one parental stock through natural selection, while individuals continue to live side by side. Selection pressures result in a divergence away from common characteristics, until such time as the two groups cannot interbreed.
Allopatry depends upon geographical splitting for speciation to occur. At some point, which may be relatively abrupt, a local group at one of the geographical fringes becomes isolated from the rest of the population by geographical barriers. The genetic diversification which follows from this formation of two separate breeding populations becomes sufficiently great that subsequent reintroduction of the two branches into the same geographical range does not result in successful interbreeding. Two separate species now exist, where formerly there was one. Both species may persist, or only one may survive to undergo further speciation. In geological time, the process of speciation may be a very rapid one, involving no more than hundreds or thousands of years. This view of a stop andgo form of evolution is at variance with the more traditional view of evolution as a long, gradual process. It is easy to imagine significant changes occurring where selection pressures are greater and species survival is more tenuous. Characteristics always appearing in a population through variation and mutation, if advantageous to the individuals’ survival, would have greater likelihood for retention in the fringe group.
The case for evolution by chromosomal change has been strengthened recently by studies on two closely related lesser apes, the gibbon and the siamang. These two apes are believed to have diverged from a common ancestral stock fifteen million years ago. They are sympatric, occupying the same tropical forests of Southeast Asia. Classified in different genera, gibbons and siamangs are not known to interbreed in the wild. In August of 1975 at the Grant Park Zoo in Atlanta, Georgia, a hybrid offspring was formed from the unexpected mating of a male gibbon and a female siamang. A second hybrid was born the following year, but it died of infection at an early age. The firstborn hybrid, now four years old, is being called a siabon. There has been much recent publicity about this firstknown hybrid between two species of apes.
The evolutionary significance of the gibbon and the siamang is not that they are capable of hybridizing. A number of other hybrid animals are known, including offspring from lions and tigers and from horses and donkeys. Their significance is that their genetic material produces very similar structural proteins, while the number of chromosomes for each species and the gene sequences on their chromosomes differ greatly. Siamangs have a diploid set of 50 chromosomes and gibbons have a full complement of 44 chromosomes. Siamang chromosomes are almost completely nonhomologous with gibbon chromosomes.
Speciation of the gibbon and the siamang from their ancestral stock appears to be the result of chromosomal changes and consequent reproductive isolation. (The siabon hybrid, as with most other hybrids, is believed to be infertile.) This speciation may well have originated rapidly and nonadaptively. If such is the case, then evolution does not merely occur by gradual accumulation of genepoint mutations.
Whether speciation takes place as a result of geographical or genetic barriers, adaptation and natural selection or nonadaptive chromosomal change, it is a process that takes place repeatedly. The traditional view of evolution is one of a linear progression of ancestors and descendants, extending from the distant past to the present. The image of a ladder is often used to represent this progression, and the ladder seldom has side rungs. Harvard biologist Stephen Jay Gould prefers
the image of a bush with numerous branching. This is a far more complicated picture of evolution, and it suggests that we can expect to find many more types of hominids in the unfolding fossil record species which very likely are not directly on the human lineage. Gould notes that archaic parental stocks may persist long after giving rise to newer species, and this is reflected in the fossil record.
Whereas scientists once constructed these ladders of descent to explain the evolution of hominid ‘lines’, there is an increasing tendency to generalize interpretations about the fossil record. David Pilbeam suggests that the more objective approach is to consider dryopithecines, ramapithecines, pongids, and hominids as groups. Modern pongids, then, are descended from the dryopithecine group. The genus
Homo
emerged from a portion of the australopithecine group, they evolving from ramapithecines or perhaps even from dryopithecines.
Ramapithecus
may or may not be on the human lineage. It was one of the first forms to exploit open grassland habitats, but it may have been an early, unsuccessful experiment.
We can talk with somewhat more assurance about the more recent evolutionary history of hominids. Johanson and White propose that the Afar apeman,
Australopithecus afarensis
, was an ancestor both to the later forms of australopithecines and to
Homo
, They feel that divergence occurred between 3.0 and 2.5 million years ago. One branch led to
Australopithecus africanus
and then to the more specialized
A. robustus
and the variant
A. boisei
. The other major branch led to
Homo habilis
, the earliest member of the genus
Homo
. Portions of the
H. habilis
population gave rise to
Homo erectus
, and part of the
H. erectus
population led to development of
Homo sapiens
. Modern man emerged as recently as 40,000 years ago.
Examination of the trends in evolution indicate that not all organ systems have evolved at the same rates. The concept that different features of an organism evolve at different rates is known as mosaic evolution. Efficient bipedal movement is a trait that appeared very early, and it is probably the single most important development in the emergence of man.