The origin of man is the most contentious aspect of evolution in the minds of many religious people because it flies in the face of biblical writings and traditions, and the concepts of evolution are incompatible with their faith. The majority of Americans believe that it is probably not true that human beings developed from earlier species. Others can grudgingly accept that flies, worms, and bacteria might have evolved but draw an emphatic line when it comes to human beings. Charles Darwin was so discomfited by the inevitable conclusions of his seminal work, The Origin of the Species, published in 1859, that he delayed inclusion of the bulk of his data about humans until 1871 when he published The Descent of Man. Inclusion of man in the evolutionary tree came as a personal struggle for Darwin. The basic argument against evolution–based on the Bible–is rooted in a condemnation of evolution science related to a dogmatic religious belief that cannot accept observable design as having any other origin than God. The logical fallacy the Creationists convey is called the fallacy of personal incredulity—“I cannot believe it; therefore it cannot be true”.
The accumulated evidence of scientific research, without invoking the supernatural–as it stands at present–offers firmly verified conclusions about the origins of man, of Homo sapiens sapiens. Human beings, anthropoid apes, monkeys, tarsiers, lemurs, and lorises are grouped into the Linnaean order Primates, a classification originally based solely on the recognition of shared anatomical similarities. More than a century after Linnaeus, scientists realized that his classification was evidence of a community of descent. The spectrum runs from the simplest members to that most modified and aberrant species which is also the most accomplished and successful, and, in fact, the only survivor of the genus Homo–Homo sapiens sapiens.
The evolutionary study of man is complicated by the daunting facts that early members of the genus, and especially of the species, were of limited number—perhaps as few as 10,000 breeding pairs, living in small groups—and their fossils are few and widespread. Also, there are a number of unrelated hominid lines making the evolutionary tree for man decidedly bushy at its beginning. The directness of lines is not only blurred by branches but by the finding of fossils that do not seem to connect to a single coherent ascent. Predictions for human evolution are also complicated by the fact that there are irregular rates of evolution because environmental changes are irregular. A good discussion of the complexities of determining which animals might be considered human ancestors and clear artistic renderings of those possible ancient humans in detail beyond the scope of the present rather brief synopsis is found in Sawyer, G.J., and Deak, V. et.al., The Last Human-A Guide to Twenty-two Species of Extinct Humans, a Peter N. Nèvraumont Book, Yale University Press, 2007. Developmental molecular biological and therefore evolutionary studies in man are necessarily limited since it is not possible (not ethical on an absolute scale) to experiment with human embryos. Visualization of gene expression patterns which have taught us a great deal about other animals will be scarce in human embryos.
Aside from the obvious appearance of humans, there are four fundamental criteria which need to be applied to any candidate from the past to qualify the subject as human: 1.Tool making ability, 2. Bipedalism, 3. Enlarged head and brain, 4. Highly dexterous tongue, effective jaw, and multiple types of teeth. Only humans have the ability to make, reproduce, and to convey information about such manufacture to other population members and to succeeding generations, about complex tools. Of all animals, only humans walk upright. The most unique attribute of humans is the skull with its markedly larger brain case holding not only an enlarged brain but a highly myelinated brain with a thick cerebral cortex. The cerebral cortex is rudimentary in primates, scarcely measurable in other mammals, and nonexistent in most other animals. The muscles and articulations of the mouth and jaw including the larynx and vocal cord apparatus afford the intricacies necessary for modulated and complicated meaningful greater range of sounds for human speech-which no other animal has, not even our nearest relative, the chimpanzee. As stated above, the eyes look forward affording both near and far vision and stereoscopic vision, all necessary for bipedalism. Modern human teeth are smaller than apes and are adapted for a frugivore [fruit eating] diet primarily with thin enamel but with sharp enough incisors and large enough canines to afford successful meat eating but not adequate for catching and holding prey, a combination not otherwise found.
Less often recognized–but nonetheless important–features of anatomy and physiology separate humans from other animals. These features lend survival and success qualities that contribute to the dominance of our species. There are eight especially important differences: 1. The brain, 2. The specialized skin which lends itself to efficient energy production and conservation and to breast feeding on the move from strong, durable nipples. Humans have more abundant and more nutritious milk for their own kind than other animals. Humans have abundant melanin in their skin to protect against ultraviolet light from the sun. 3. Jaws, 4. Efficient sex, 5. Energy producing and conserving gut and its biotome. The human microbiome—the collective genomes of a microbial community—has had to respond to its constantly changing environment with dramatic changes in genomic structure. Humans are exposed to incredibly varied environments; human guts come into contact with a diet more complex, variable, and changing than any other animal. Our microbiome has a truly incredible capacity to evolve—to make and to pass on heritable DNA changes that are incorporated into the collective genome and to do so quickly and safely. This capacity of our gut microbiome is a dynamic contribution to human adaptation, survival, and persistence and is a significant evolutionary advantage
for humans. 6. The ability to communicate, especially in abstractions and creativity and to create beneficial and protective social interconnections, and 7. Shorter gestation period than in other animals of comparable size which allows a smaller fetal head and therefore a smaller female pelvis, and 8. Unfused cranial bones that are collapsible upon each other to permit further reduction in head size and allows easier birth and a lower infant mortality.
The ability of humans to run and thereby to catch prey and to avoid predators is based on 27 special human features including such evolution endowments not shared with other animals as:
- powerful gluteal muscles
- stiff feet
- short toes
- long Achilles tendon
- longer legs
- generally elongate morphology
- head stabilization including a powerful nuchal ligament
- short snout
- the right kind of skin for heat dissipation and adaptation to differing temperatures
- humans have been major beneficiaries of those evolutionary changes which have made a significant contribution to our ability to have large, complex, facile, high energy requiring, and extremely useful brains.
Predators, such as cheetahs, can run faster than humans and other predators and prey species in a sprint, but they can only sustain the chase for about one hundred yards or so before having to stop—a deucedly inconvenient situation when you are being chased by a lion or when you have used up your last ounce of energy going after prey and now must face the specter of starvation. Animals other than humans have predominantly apocrine glands in their skin which produce secretions that are high in fat content, and a good lubricant, but does not bathe the body in evaporating water. All of them must eventually stop or go into thermal shutdown. Those animals must depend on panting, an inefficient method in comparison to effective sweating. Human morphology and biochemistry enables marathon running, an advantage over sprinting.
The ability to communicate—linked to thought–is of indisputable value to humans. The ability did not come in one miraculous event, nor was it present until fairly recently in human evolution. Development of the brain favored primates in general and particularly in the evolution of crucial improvements in communication culminating in the tremendous advantage afforded by the brain and speech apparatus structure in humans. There appears to have been a fairly rapid burst of increased brain size about 1.8 MYA and another between 600 and 150 KYA during periods of severely changing climate. Brain size doubled over some 50,000 hominin generations during those climatic conditions. Tracing gene mutations back to the time that chimps and humans separated, it was noted that humans underwent rapid changes about 200 KYA which spread throughout our species. Intracranial size expanded significantly 50 KYA at about the same time the FOXP2 gene mutated and facilitated speech.
Further specialization in humans led to the formation of the planum temporale which is crucial to spoken language, gestural communication, and music. These specialized areas and hemispheral dominance for various functions are present in apes, but have come to fruition in humans. In evolutionary terms, it must be presumed that apes and humans have a common ancestor with such defined neuroanatomy that led from the common and basic pre-ape, pre-hominid condition to the specializations found in the two major divisions of the hominins and hominids. Evolutionary “tinkering” in the microanatomy of human brains over hundreds of thousands of years altered the number, arrangements, and connectivity of neurons.