The First Hominid

HUMAN EVOLUTION

Humans are primates. Physical and genetic similarities show that the modern human species, Homo sapiens,
has a very close relationship to another group of primate species, the apes. Humans and the
so-called great apes (large apes) of Africa—chimpanzees (including bonobos, or so-called pygmy chimpanzees) and gorillas—share a common ancestor that lived sometime between 8 million and 6 million years ago. The earliest
humans evolved in Africa, and much of human evolution occurred on that continent. The fossils of early
humans who lived between 6 million and 2 million years ago come entirely from Africa.


WHO WAS THE FIRST HOMINID?: Time travel would make everything so much easier. Imagine that you could drop down by an African lake some 7 million years ago and watch the parade of aardvarks, antelopes, and elephants pass by until, sooner or later, you caught sight of a group of apes. They'd probably look something like chimpanzees—about the same height, with the same coat of hair—but their flat faces and the other odd proportions of their bodies would indicate that they belong to a different species. Perhaps they would turn your way and look you in the eye—a gaze from your most distant hominid ancestors, the first primates to split off from the other apes and begin the family that produced us. Such are the daydreams paleoanthropologists indulge in as they endure blazing heat, merciless sandstorms, and years of fruitless fieldwork.

If the earliest hominids were anything like chimps, bonobos, and other living apes, each species may have numbered in the hundreds of thousands, even millions. But few left fossils behind. Most of their bones were scavenged and scattered by hyenas or other animals, and what little remained rotted. When it comes to early hominids, paleoanthropologists have to make do with a few teeth or skull fragments.

Yet paleoanthropologists are learning a lot about our origins. Not long ago, the oldest known hominid was Australopithecus afarensis, a species that walked the savannas of East Africa around 3.6 million years ago and is best known from one well-preserved female skeleton found in Ethiopia in 1974 and nicknamed Lucy. In recent years, paleoanthropologists have found perhaps as many as five species that are older than A. afarensis—in some cases much older. Michel Brunet of the University of Poitiers, in France, and his team of explorers announced that amid the sand dunes of the Sahara they had found a species between 6 million and 7 million years old: Sahelanthropus tchadensis.

These new fossils have thrown cherished orthodoxies into question. "We saw human evolution as a nice, straight line," says Leslie Aiello of University College London. Now some researchers are arguing that human evolution looked more like a bush, with lots of species branching off in different directions.

No new orthodoxy has gained enough strength yet to take over the old one. Instead, there's lots of debate. Some paleoanthropologists, for example, have declared Sahelanthropus to be on the line that led to gorillas, not humans. "That's crazy," replies Brunet, who points to small teeth and other key traits that link the creature with hominids rather than apes. But while Brunet is confident he has discovered the oldest known hominid, he doesn't think it's possible yet to make grand pronouncements about the shape of the hominid tree and its various branches. "You can't say that it's bushy," he says. "Maybe it is; we don't know. Our story has just doubled in time, and we're just beginning to understand it."

<< Back to Human Evolution; Evolution of Early Primates; Primates are remarkably recent animals. Most animal species flourished and became extinct long before the first monkeys and their prosimian ancestors evolved. While the earth is about 4.54 billion years old and the first life dates to at least 3.5 billion years ago, the first primates did not appear until around 60 million years ago. That was after the dinosaurs had become extinct.

The earliest ones are found in the fossil record dating to 60 million years ago. Prosimians thrived during the Paleocene and Eocene Epochs. There were no monkeys or apes for them to compete with yet. By the time of the transition to the Oligocene Epoch, monkeys had begun to evolve from prosimians and became the dominant primates. Many of the prosimian species became extinct probably as a consequence.

By the early Miocene Epoch, apes had evolved from monkeys and displaced them from many environments. In the late Miocene, 5-6 million years ago, the evolutionary line leading to hominids finally became distinct. This hominid line included our direct ancestors. So, the proliferation of mammalian forms from the end of the Cretaceous period (ca. 65 Mya) coincided with the extinction of many of the other life forms which roamed over the earth at that time (including dinosaurs). The depopulation of the planet likely opened up many new niches, which accounts for the rapid increase in mammalian species following the Cretaceous-Tertiary boundary. Primates - or their tree shrewlike progenitors - were one of these opportunistic, niche-filling mammals.

However, given that a great many mammalian types emerged at this time, how is it that primates acquired the peculiar battery characteristics which would prove so instrumental in the later success of the order?; Three Hypotheses; An early theory of primate evolution, proposed by F. W. Jones (1916), relates the primate characteristics of grasping hands and feet, orbital frontation (increased binocularity of vision), and enhanced cognitive processing capacity to the challenge of arboreal life. Generally known as the Arboreal Theory of primate evolution, this theory explains the enhanced visual-motor systems and cognitive elaboration in primates as products of arboreal life. However, one major weakness in the argument for the Arboreal Theory is the fact that arboreal mammals exist which do not possess any of these traits, and yet are very successful.

An example of this would be tree squirrels, which have very little visual overlap, non-opposable digits, and smallish brains; however, as any walk through a park may attest, squirrels are quite successful in their own right. As response to this, in 1974, Duke University primatologist Matt Cartmill proposed a new theory - the Visual Predation Theory - to account for the evolution of primate characteristics. Instead, he suggested that arboreal predatory behavior accounted for the grasping hands and feet, and particularly the increased visual overlap and brain size (both characteristics observed in terrestrial predators, such as large cats).

However, a weakness in the Visual Predation Hypothesis lies in the fact that prosimians, considered to be closer to the ancestral form of all primates, exhibit lower reliance on visual information for locomotion and predation; rather, they emphasize olfactory and auditory cues in the pursuit of prey. In light of this fact, Washington University primatologist Robert Sussman proposed the Mixed Diet Theory - namely, that an increased exploitation of angiosperms (flowering plants) selected for modern primate characteristics. Enhanced visual acuity, color vision, and characteristics amenable to exploiting terminal branch resources all allowed for efficient acquisition of a resource with an angiosperm-like distribution. Additionally, the emergence of flowering plants in the Paleocene roughly coincides with the emergence of the earliest primate ancestors.; The First Anthropoids; While the earliest potential anthropoid primates may have emerged close to 54 million years ago, the first unambiguous remains date back to approximately 36 Mya, from a region called the Fayum in Egypt. One of the more complete fossils from the Fayum is a monkey named Catopithecus brownii, a diurnal (active during the day), arboreal quadrupeds, and probably fed primarily on insects. One characteristic linking C. brownii with modern day anthropoids is the shared dental formula between it and all modern Old World monkeys. Since New World monkeys have different dentitions, C. brownii is not likely to be ancestral to any of the New World species. Later fossil primates from the Fayum - including Aegyptopithecus zeuxis, pictured at left - are grouped into parapithecoids and propliopithecoids. The propliopithecoids are considered to be possible progenitors of modern anthropoid primates.

<< Back to Human Evolution; The New World Monkey Enigma; The puzzling ancestry of New World monkeys derives from a low abundance of fossil material, as well as limitations based on knowledge of continental drift. Similar biological diversity and geology suggest that the continent of South America was once a part of Africa. South America eventually broke off of the African mainland (through a geologic process called continental drift), and over the course of millions of years, migrated to its present position. Unfortunately, the separation between South America and Africa occurred over 100 Mya ago, which is much earlier than the first primate fossils on either continent. This leads to several possible scenarios for the ancestry of New World primates:

1. African anthropoids crossed the Atlantic somehow, and radiated into new habitats upon reaching South America.

2. North American primate forms gave rise to the current New World species.

3. African anthropoids emerged earlier than fossils suggest, and rafted across the Atlantic when it was much smaller.

The problem with the first interpretation is the tremendous distance involved in a trans-Atlantic journey (by the late Oligocene it was already over 2000 miles). Large chunks of land mass have been known to break off and essentially "float" across large bodies of water as living cargo vessels, although evidence has yet to emerge in support for this hypothesis on the origin of New World monkeys. The second interpretation is problematic in the sense that North America does not have any evidence of anthropoid species. Thus, descent from a prosimian-like common ancestor would require a remarkable number of evolutionary convergences between New World and Old World anthropoids, a concession many systematists find difficult to accept. The third possibility provides a compromise of sorts between the first two explanations, suggesting that an ancestral primate form might have rafted across the Atlantic, but when it was much smaller. This would push back the divergence dates! for anthropoid primates beyond those supported by the fossil record, but as an old anthropological adage goes, "absence of evidence is not evidence of absence".

Estimations of error in fossil sampling suggest that anthropoids may have actually emerged as early as 52 Mya. An earlier emergence of anthropoid primates might allow for a transoceanic voyage before the distance between South America and Africa became too large.; Emergence of the Hominoids; Tracing Human Evolution to its Roots; Molecular evidence suggests that the human line split from the chimp line only about 6 million years ago. Some recent fossil finds date from between 4.1 and 6 million years ago, placing them very close to that evolutionary divergence. Evidence from fossil animals, plants, and soils associated with these early hominids indicate that the environment in which they died was quite densely forested, prompting a reassessment of some of the hypotheses that have been suggested for the origins of human bipedalism.; A series of new fossil discoveries in the 1990s generated great interest among scientists involved in research into human origins, pushing the date for the beginnings of bipedalism back to more than 4 million years before the present -- and into a rather different context from the savannah environment which earlier research had suggested. In 1994, an international team led by paleontologist Tim White announced that they had found a new species of hominid in Ethiopia, dating from 4.4 million years ago. So distinctive were these fossils that the researchers decided that they came not only from a new species, but from a new genus as well, and gave them the name Ardipithecus ramidus. ("Ardi" means ground or floor in the local Afar language, and "ramid" means "root.")

The teeth of this species are more apelike than those of Australopithecus afarensis fossils like Lucy, and its brain was small. The intriguing question, though, is how it got around. Did Ardithecus ramidus walk bipedally, like modern humans?

The fossil was encased in a hard matrix of rock, and the painstaking process of preparing and analyzing it is not yet complete. But some features of the skull suggest that it had some form of upright posture.

If A. ramidus was indeed a biped, a few of the hypotheses that have been suggested for the origin of bipedalism may have to be reassessed. Most of these ideas were based on earlier research which suggested that upright walking evolved at a time when open grasslands were replacing dense forests in the region. But A. ramidus was found with fossil seeds, plants, and animals which show that the environment was densely wooded.

Less than a year later, a team led by Maeve Leakey and her colleagues uncovered a new species of hominid, which they named Australopithecus anamensis (from "anam," the word for "lake" in the local Turkana language). The fossils were found at two sites in Kenya, Kanapoi and Allia Bay, in the Lake Turkana region. They were found in the sediments of an ancient lake, along with animal fossils that suggest that the environment was a river and gallery forest, grading into more open woodland. Other fossils from Kanapoi include fish and reptiles. At 4.1 million years old, this new find predates Lucy's species, A. afarensis, by more than half a million years, and features of the leg bones definitely indicate that A. anamensis stood upright and walked bipedally.

In 2000, a team led by Brigitte Senut and Martin Pickford reported a 6-million-year-old fossil that they named Orrorin tungensis. While its discoverers are confident that Orrorin is a hominid, not every expert is convinced by the evidence presented so far. Some feel that it may prove to be a chimp ancestor -- which in itself would be exciting, since so few fossil apes are known -- or even the common ancestor of chimps and humans.

More recently still, Yohannes Haile Selassie, a colleague of Tim White, announced the discovery of another fossil which he named Ardipithecus ramidus kadabba, a distinct sub-species of A. ramidus. "Kadabba" means "oldest ancestor" in Afar, and indeed, at between 5.2 and 5.8 million years, A. r. kadabba, like Orrorin, is very close to the date of the split between chimps and humans established by molecular evidence. The fossil animals, plants, and soils at the site show that A. r. kadabba lived in a forested landscape, and details of a toe bone show that it was bipedal, although its gait was probably quite different from anything seen today.

Until more fossils of A. r. kadabba and Orrorin are found and further analysis is completed, it will not be clear how they moved around or just where they fit into the overall picture of human evolution. But there is no doubt that with these latest finds we are getting very close to the root of the human family tree.; When Humans and Chimps Split; A new study of genes in humans and chimpanzees pins down with greater accuracy when the two species split from one.

The evolutionary divergence occurred between 5 million and 7 million years ago, an estimate that improves on the previous range of 3 million to 13 million years in the past. Modern chimps are the closest animal relative to humans. Knowing when the two split has implication both for understanding how quickly evolution works and for imagining the likelihood of intelligent beings elsewhere in the universe, researchers said today.

"There is considerable interest in knowing when we diverged from our closest relative among animal species," said Sudhir Kumar of Arizona State University. "This divergence time also has considerable importance because it is used to establish how fast genes mutate in humans and to date the historical spread of our species around the globe."

Kumar’s team used a recently developed method in genetic sequencing to make the most comprehensive comparison to date of genes from humans, chimps, macaque monkeys and rats. They examined the number of mutations in the DNA sequence of each species to estimate its rate of evolutionary change.

"We can conclude that humans and chimpanzees probably last shared a common ancestor between five and seven million years ago," said research team member Blair Hedges, an astrobiologist at Penn State. "Knowing the timescale of human evolution, and how we changed through time in relation to our environment, could provide valuable clues for understanding—in a more general sense—the evolution of intelligent life."

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