Hominids and their History

 

Modern Evolution, the New Kind

Our modern understanding of how evolution works is more clear and precise than it was before gene sequencing, and scientific proof of certain concepts is easier to get. Radio carbon dating, microbiology, and new forensic techniques have helped us understand our history. Despite our culture’s expanded understanding of biology and history of life, the ideas of variation, selection, survival, and extinction are still basically the same as the ideas outlined in Darwin’s Origin of Species.

During the life of a multi-celled organism cells will die off or become damaged. New cells are created through the most basic form of reproduction, mitosis. In most cases each cell has the exact same DNA sequence as the original cell but occasionally the copy will be slightly different from the original. This represents the first source of variation: “mutations in mitosis”. If the resulting difference in the organism is not instantly fatal, it can either be helpful to that creature’s survival and reproduction or a hindrance. If the resulting difference is a hindrance to survival and/or reproduction, the statistical probability of the continuation of those traits in that population goes down. If the resulting difference is helpful to survival and/or reproduction, the statistical probability of the continuation of those traits in that population goes up. This tendency in nature represents the most basic form of selection: “environmental selection”. Predators, pathogens, toxins, and temperature can also be agents of selection by killing off some members of a population but not others. These forces of environmental selection do not act constantly or predictably on a population but populations change more rapidly or become extinct when more selectors act on a population. When there is more genetic diversity, there is less mutation. When there is less genetic diversity, there is more mutation. This quality in nature allows some populations to survive, even when agents of selection change. Constant change, motion, and growth, are the reasons behind the diversity of life on our planet.

In the 1850’s, Charles Darwin described the forces of natural selection in terms of how they acted on presently living animals. For example, a bird who could access food better because of a naturally occurring change in beak shape would have a slight advantage in it’s environment over other birds. This advantage would raise the birds’ statistical chances of living long and reproducing well. Thus the beak variation would increase in that particular bird population. Agents of environmental selection like predators could influence color of the bird by killing off more of the brighter colored variations. Sexual selection could result in things like singing by advertising and exaggerating a male’s attractiveness. If the population of birds was to move or if the climate changed, different selectors would be present and their evolution rate could change. Darwin knew from geology that time is longer than most people can understand, and that there really was room in the over-all timeline for trillions of little variations and selections like these to turn one organism into all known earth life.

Sex with multicellular organisms

For most of the history of earth life, sex did not exist. The poor creatures had to clone themselves asexually. There is less chance for genetic variation in a system without sex because random mutations are the only real source of variation. Evolution can still happen but it takes longer. When this slow evolution eventually led to creatures who could exchange genetic information, this multiplied the chances of “mutations through mitosis” and brought about the possibility of gene flow. Gene flow is a force of variation that occurs when two slightly different populations of the same species seperate, isolate, evolve independently, then come together. Reproduction involving two distinct genders of the same species eventually came about. Variations and adaptations from 2 genealogies are better than one. The emergence of sex introduced another force of selection: “sexual selection”. Sexual selection selects traits in a population through attraction. Large organisms who reproduce sexually produce a special kind of cell called a sex cell. The sex cell exists for the exchange of genetic information during sexual reproduction and must reproduce itself by splitting into clones just like any cell. This is called meiosis, and it is distinct from regular mitosis in the fact that only half the genetic sequence needed for another organism is left in the 4 daughter cells. The meiosis of sex cells introduces yet another source of even more genetic variation because the copies of sex cells can have copy errors just like in mitosis. The part of the gene sequence of an individual that is passed on through sex is left to random chance and not all of an individuals gene traits will show in all circumstances. The science of how genetic traits show in reproduction was pioneered by Mendel during Darwin’s lifetime but the study became much easier with the scientific discovery of DNA, RNA, genomes, and gene sequences. The emergence of gene exchange and later sexual reproduction must have raised the top possible rate of evolution in Earth life. If it were not beneficial, forces of selection would have removed it from the gene pool. More sources of variation and more sources of selection increase the effectiveness of evolution. Sex achieves both.

How primate evolution works

The oversimplified concept that “man evolved from apes” could be more accurately stated by saying “Creatures with some similarities to modern prosimians lived in the eocene period when the globe was warmer and more thickly forested. The morphology of these prosimians had evolutionary advantages. Monkeys and apes became common by 33-29 million years ago, and by 18 million years ago, there were many large apes. Since we are large apes and have some idea how evolution works, we know that one of these large apes must be our ancestor.” Our understanding of human origin is complicated by the fact that there were many distinct species of bipedal apes and at least 7 periods of catastrophic climate change between the earliest known bipedal ape and the earliest known anatomically modern human. Questions like “Why are there still apes?”, “could this be the missing link?” and “Why didn’t the chimps evolve into humans?” only make sense if we misunderstand the nature of evolution. Evolution is never linear or progressive. It never starts or stops. When the rate of change in a species slows down or speeds up, it is because of agents of selection.

Primate Morphology and Evolution

The fossil forms that have been found give tantalizing clues but not a complete detailed story. Ancient primates are better understood by comparing them to modern ones. The study of ancient primates and prosimians is relevant to the study of humans. It is relevant because the traits that nature selected in our evolutionary predecessors are what determine the kind of creature a human being is.

The first prosimians that we know about existed around 40 million years ago. These were squirrel-like but they had certain characteristics that would be built upon in all primates. They had flatter faces, giving them a downward peripheral vision. Their eyes were well developed and positioned for stereoscopic, frontward vision. They had 4 nimble, opposable, dextrous limbs. They had a peculiar arboreal locomotion that involved leaping, judging distances, and catching the next tree with their hands. This type of locomotion favors traits seen in all primates today including us. It is very likely that they lived in social groups. This capacity to form social bonds, tribes, and societies is important in all the primates we know about and especially in modern humans.

Similar Simians and Primitive Primates

By 33 million years ago, primates resembling modern monkeys were abundant. Warm climates and dense jungles covered much of the globe. Aegyptopithecus, micropithecus, and others like them populated the dense forests and subtropical . They were quadrupedal, with prehensile tails like modern monkeys, but their dentation and joints were different. They are sometimes referred to as dental apes because despite their monkey-like bodies, their teeth were more similar to modern apes than to modern monkeys. This tells us that their diets and locomotion where not identical to modern quadrupedal tree monkeys.

By 23 million years ago, miocene tree apes like pro-consul had come into being. Pro-consul was tail-less, and would be called a “primitive ape” if it lived today. It’s head was like that of a modern chimp but it’s body was more macaque-like. It walked on top of branches. There were many ape-like monkeys alive in the miocene 12 to 10 million years ago. Their bodies worked much like the bodies of strictly bipedal arboreal monkeys but many were tail-less and had heads like modern apes. Sivapithecus was very much like an orangutan and may be related to them but the shoulder joints are not as flexible. Later dryopithecus is important because we know that it was a brachiator. This means that they could swing below branches and easily reach out in front of themselves. 7 to 8 million years ago, another miocene ape,oreopithecus was suspensory swinging. Its specialized leafy diet made it vulnerable to climate change. An enormous, gorrilla-like ape, gigantopithecus also lived during this time. The large size of this ape tells us that their food source must have been abundant. The gigantopithecus had a large sagital crest, grinding flat molars (nothing like dentation of modern gorillas), and wide zygomatic arches. This tells us that whatever it ate required a lot of grinding and chewing. Remains that can be gene-sequenced have not been found for this time period, so theoretical relationships of sivapithecus to orangs and pro-consul to chimps are based on morphology. The existence of these strange apes, and others like them, made human life possible. Their evolutionary success and diversity would later lead to diversity in bipedal apes.

During the end of the miocene, earth’s climate was changing drastically. Many species of land animals would have gone extinct. The climate changes of the late miocene were not like our current climate changes. These upheavals created the ice caps at the poles and created the Alps. Our thickly forested planet full of ideal habitat for creatures like oreopithecus was rapidly de-foresting.

Modern science is reasonably certain that bipedalism came about during this time, 6 to 7 million years ago. It must have worked well in many environments because a diverse range of bipedal primates thrived for millions of years. Remains of Ardipiththecenes have been found from around 5.8 to 4.4 million years ago, These pre-australopithecenes and others had ape-like heads but human-like gluteals. Some, like ardipithecus kadabba, were spending partial time in the trees like modern chimpanzees, but the hip bones from this time period suggest bipedality. Skull fragments showing the position of the foramen magnum have not been found from this time period so the nature of bipedalism as it existed then is still debated. Teeth that have been found suggest diets similar to ours, than that of modern ape’s. The fossil record from this period is not plentiful. These pre-australopithecenes and later bipedal primates are called “hominids” because early anthropologists assumed that human-ness and bipedality were related.

Aussies

More complete remains of bipedal apes have been found from a pliocene period of 1.8 to 4 million years ago. Australopithecus phenotypes (in general) have chimp-like heads, human-like feet (some have diverging toes, but not fully opposable toethumbs), long arms with hands, and varied diets. It is nearly impossible to infer anything about behavior or psychology from braincase endocasts, but it is likely that their language and societies were like that of modern chimps. Most have dentation that suggests omnivorous diets. They were capable of eating hard nuts, fibrous grasses, and meats. The famous remains of “Lucy” are important because of their completeness and age. Lucy is representative of A.afarensis from around 3 and a half million years ago. The remains were so complete that they were able to sample the genome. A.afarensis is a predecessor to all the later diverse hominid forms and every currently living hominid. A. afarensis had a more forward sweeping face than some previous forms and probably had a gorilla-like neck. It’s teeth are like that of hominids and we have fossilized footprints to verify that they walked like A.M.H. Complete craniums of A.africanus have been found on the African continent. These had thick brow ridges and chimp-like jaws. The posterior skull of these creatures is interesting because they are rounded and lack a crest. Their zygomatic arches leave plenty room for jaw muscles in their ape-like faces but the backs of their heads are very human. A. robustus, boisei, and aethiopithicus had far heavier dentation and large molars, with fuller sagital crests for strong jaw muscles. Teeth like theirs do not exist in any modern primate, so the food available in their environment must have been unlike the diets of any modern primate. Fossil forms like these robust australopithecenes seem more primitive to us but the timeline puts them as co-existing with, or post dating more human-like forms like A.africanus by over a million years. Their dentation suggests specialized herbivorous diets. Specialized diets can be an evolutionary advantage when a particular food-source is plentiful; but during times of climate change, creatures with selective diets can be selected out of the gene pool when that food source becomes scarce.

Recently, part of a maxillary plate, teeth, brow ridge, and garbage dumps of Australopithicus garhi have been found from 2 and a half million years ago. A.garhi had shorter arms, like A.M.H. This australopithecine is exciting because we know they made stone tools and liked to live near water, like A.M.H. Their diets were more varied than the late, robust australopithecenes. They had bicuspids similar to ours instead of the 5-y “tricuspids” seen in other bipedal apes. Scraped animal bones found at their sites tell us that they butchered meat with stone tools. It is likely that they made tools for processing a wide variety of foods from different sources.

Homos- 1.8 million to 400,000 years ago

As climate change began to kill off land life that had lived for millions of years, most species of the bipedal apes also became extinct. Bipedal hominids from this period looked very different from the australopithicenes. Large, round braincases, taller foreheads, smaller brow ridges, smaller teeth, and smaller faces separate the homo genus from the australopithicenes. It is hard to scientifically prove what kinds of cognitive changes correspond with homo’s larger brains but we know that they made tools and didn’t immediately die off. It is likely that they communicated more like we do and less like modern apes do. Homo species from the late pliocene, up until the earliest known A.M.H. around 300,000 years ago, demonstrate a diversity of forms that has been non-existent for all of recorded human history. They even lived alongside A.robustus. The situation in our time, where one hominid species covers the globe and there are a few species of large apes, is rare in primate history. There are several times in pre-human history when hominids as different as chimps to orangutans would have co-existed. All the homos were tool makers and tool users. The creation of tools requires a type of psychology that is uniquely human. Evidence suggests that the social organization and larger brain sizes of the homo became an evolutionary advantage. Glaciers moved toward the equator then receded again at least 4 separate times. Most primates at the time could only live in warm arboreal climates. These periods of glaciation would eventually eliminate nearly all primate species on the planet. The primates who could live through this were clever, organized tool-makers. H.habilis and h.erectus survived whatever selectors ended the australopithicenes. H.erectus was successful enough that they had to emigrate and find new lands. Their culture and communication made pack-hunting, technology, and increasingly diverse diets possible. The range of h.erectus was less hindered by cold climates. Complex cognitive abilities endowed these populations with the ability to change diets, behavior, location, or social organization instantly and at will. Earlier australopithecenes would have needed evolution to make such drastic changes. The wide range and long existence of h.erectus made morphological and cultural diversity possible. Some Homo erectus skulls show enormous brow ridges. Some had rounded, ape-like jaws. Over time, evolution selected for bigger brow ridges, more gracile bodies, smaller teeth, and more complex cognitive abilities. Populations of h.erectus from different parts of the world would have looked very different. Modern humans, by comparison, show almost no morphological diversity even with our enormous numbers. This is probably because the last near-extinction of h.sapiens during the ice age had reduced the human population to around 600 breeding individuals. By 300,000 years ago, H.erectus had already spread to much of the habitable globe. Their decendants, H.neanderthalensis and H.heidelbergensis where living on the European continent when another, later, hominid migration wave came out of the African continent.

Anatomically Modern Humans and Trolls

These early h.sapiens, like cro-magnon man, had more diverse diets and larger tribes than the h.neanderthalensis did. Neanderthal and sapiens both wore clothes, hunted and gathered in organized groups, buried their dead ceremoniously, made tools, and made fires. Neanderthal’s brains were larger than sapiens but dental analysis of h.neanderthalensis children suggests that they reached adulthood sooner. Their trash dumps suggest smaller tribe size and evidence of neanderthal art is still disputed (rare or non-existent). These findings imply that their cognitive abilities were different from sapiens, who made art, used body paints, and probably had complex language. The real verifiable difference between neanderthal and sapiens is biological. The neanderthal was incredibly tough and strong compared to early sapiens, and those early sapiens where stronger and tougher than us. Neanderthal was better suited to cold climates, but their unusual capabilities required large, daily amounts of meat. Like the later australopithecus, their diets had become more specialized. Periods of climate change can change food sources and the spread of pathogens. If a late immigration wave of hominids crossed continents 260,000 years ago, other animals and pathogens would have also crossed continental borders. An influx of other hominids, non-native wildlife, and new diseases would have been hard on the native hominids of Europe. Early sapiens were built lighter and had more diverse diets. Evidence suggests that the early sapiens ate fish, and this would have given them an advantage over the more carnivorous neanderthal if population numbers of large land animals changed and we know from the fossil record that they did. Different theories have been suggested for the disappearance of h.neanderthal, including interbreeding, war, and starvation. Disappearance of their main food sources, through migration, disease, climate change, or competition with sapiens was the biggest factor in their decline.

From the h.sapiens of 50,000 years ago to the present, humans have seen a few negligible changes in morphology and large changes in technology and social organization. The abilities that gave us an evolutionary advantage during catastrophic climate changes and the last ice age have led to huge populations in the recent 5000 years of relative climatic stability. While most modern humans feel that they are a pinnacle of evolutionary success, it is still unproven if our species will be as successful as h.erectus or h.neanderthalensis.

 

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