The hominid family tree has 7 widely acknowledged genera: Sahelanthropus, Orrorin, Ardipithecus, Kenyanthropus, Paranthropus, Australopithecus, and Homo. There were doubtlessly more.
Hominins were born when grasses were on the rise. ~ American paleoanthropologist Ann Gibbons
Climate was a likely driver of the radiation of early hominids, at a time of warming, when forests thinned. Social factors may have also been instrumental, but there is no evidence of this, nor would there be even if it were so.
The fossils of the earliest hominids were found in Europe. Living 9.6–7.2 MYA, Ouranopithecus appears a mixture of ape and hominid.
The fossils of the other early hominids were dug in central east Africa. Later finds range more widely, to the west and south.
Teeth are the most common fossils, because they withstand the punishments of time better than any other body part. Dental remains, by the shape of various teeth and their wear patterns, yield indications of diet.
To estimate evolution, early hominids have been compared to our closest relatives: chimpanzees and gorillas. This apples-and-oranges comparison is complicated by lacking any significant fossil record of apes.
Once asked if hominid fossils spoke to him, Kenyan fossil hunter Kamoya Kimera replied: “yes, but you can’t understand them!”
Sahelanthropus tchadensis (“the Sahel man from Chad”) (7–5.6 MYA). Few fossil specimens are known: a partial cranium, 5 pieces of jaw, and several teeth. No other body parts have been found.
Sahelanthropus probably walked upright. The braincase, at 320–380 cm2, appears like extant chimpanzees; almost 4 times smaller than modern humans.
Sahelanthropus‘s huge brow ridges and face resemble the face of Homo, as does the small canine teeth. But Sahelanthropus‘ tooth enamel was very thick: suited for chewing vegetation, nuts, and tubers. This combination has not been found in fossil apes, nor seen in later hominids.
Sahelanthropus may represent a common ancestor of humans and chimps, or an ancestor to neither. Some cranial characteristics resemble those of Miocene apes. Other features are more like later hominids. Sahelanthropus‘ fossils are separated from both these groups by geography and time, so Sahelanthropus as its own genus is justified.
Precious few ancestors of chimps or gorillas have been found. The upshot: the inclusion of Sahelanthropus in the hominid family tree is problematic, and its place uncertain.
Orrorin tugenensis (“original man of the Tugen hills”) (6.2–5.6 MYA) presaged later hominids. The thickness of a found femur (thigh bone), as well as its hip socket, identifies Orrorin as bipedal, though the orientation and shape of the toe suggest a grasping foot. The shapes of the upper arm bone and slightly curved finger bone intimates that the upper arms were weight-bearing. Orrorin looks to have been a tree climber as well as walking upright.
Found teeth suggest a diet of fruit and seeds: large upper front teeth, reduced apelike pointed canines, microdont (smaller) post-canines like modern humans, and low cusped molars.
The location of fossils found indicate that Orrorin lived in dry evergreen forests, lakeside woodlands, and wet grasslands – not the savanna long assumed as the progenitor environment of hominins.
Orrorin arose 3 MYA earlier than Australopithecus afarensis, but Orrorin‘s anatomy is closer to modern humans. For one, Orrorin‘s hand indicates a precision grip, more capable of fine manipulation than many later hominids.
If Orrorin is the ancestor to modern humans, australopiths may be a side branch of the hominin tree. The lineage to humans remains unclear and contentious.
<em>Ardipithecus (the Afar word for “basal family ancestor”) (5.8–4.3 MYA) is a genus with 2 known species: Ar. kadabba and Ar. ramidus.
Little is known of the earlier Ar. kadabba, as only teeth and pieces of skeletal bones have been recovered, dated 5.6 MYA. Ar. kadabba lived in a habitat mix of woodland and grassland, with small lakes, swamps, and springs.
By contrast, a variety of Ar. ramidus fossils have been found, altogether comprising almost every part of the skeleton. The single most complete fossilized skeletal remains were of a female that lived 4.4 MYA, named Ardi.
Ar. ramidus had a modest stature: around 1.2 meters, and modest brain size (300–350 cm2). There was little sexual dimorphism: males and females were similarly sized, unlike chimpanzees. This suggests that that competition between males was minimal. Sexual dimorphism commonly occurs when males compete for sex.
Males and females may have both taken part in food gathering and taking care of offspring. Sociality may not have been as ritualized or segregated as it is with some other primates, notably baboons.
Many physical features suggest Ar. ramidus both walked upright and swung from limb to limb. Chimp feet are good for grasping trees. Ar. ramidus feet were better for bipedal locomotion but the arms and hands are consistent with clambering in trees.
Ar. ramidus lived in the woods, preferring the trees to the open plains of the savanna. They lived mostly during a time when drought was rare.
The Ar. ramidus teeth found are not sufficiently differentiated to indicate limited dietary preferences. This suggests an omnivore or broad vegetarian diet, as the molar enamel is thicker than that of chimps, but thinner than later hominins.
It is unlikely that Ar. ramidus ate hard foods, such as tubers and nuts, which require thick enamel for heavy chewing. Ar. ramidus probably enjoyed fruits, leaves, and softer vegetables, and perhaps insects, eggs, and small animals.
Plant-eating baboons lived in close association with the australopithecines and probably helped form an effective early warning system against predators such as big cats, especially leopards. ~ English educator Douglas Palmer
Australopithecus (“Southern ape from the lake”) (4.2–1.8 MYA) is a relatively long-lived genus of hominids, with considerable species diversity. There were at least 8 distinct australopiths. All have been found in Africa.
Most australopiths dieted on fruit, grasses, sedges, vegetables, and tubers. That an australopith evolved into the first of the Homo genus is generally accepted.
In most mammals, females stay in their natal community, while males disperse after adolescence to avoid inbreeding. Among the apes, gorillas follow this pattern. In contrast, chimpanzees and early hominids practiced the opposite. For chimps and their close evolutionary descendants, including australopiths, cohesive male bonding required growing up together. For the sake of communally defending territory, post-adolescent females were forced to disperse to find new lives. Doubtlessly males did not mind taking them into their tribe.
Four million years ago, if you broke your jaw, it was probably a fatal injury. You wouldn’t be able to chew food. You’d just starve to death. ~ American evolutionary biologist David Carrier
Early hominids losing the sharp canine teeth of their primate ancestors took much of the bite out of biting an opponent. There was compensation at hand.
Australopiths evolved numerous traits that endowed greater fighting ability, including a hand that afforded formation of a fist. This turned a delicate musculoskeletal system into a club.
Contemporaneously, the faces of males diverged from those of females. The facial bones that differ most are those that strengthened to protect the face from injury during fist fights.
The function of the fist was reinforced by bipedality. Unlike other primates, apes and hominids walk on their heels. This body posture lends extra punching power, and suggests physical conflict was instrumental in hominin descent.
Later Homo descent traded weaker muscles for comity. Human arm and upper body strength is much less than australopiths. This reduction in damaging ability afforded more gracile bodies, even as facial bone differences still characterize human sexual dimorphism.
Au. anamensis first made the scene 4.2 MYA, lasting to 3.9 MYA. Its thick enamel teeth suggest a diet of nuts, seeds, fruits, and leaves. Au. anamensis was a tree climber. How bipedal it was remains unsettled.
Au. afarensis (3.9–2.9 MYA) likely descended from Au. anamensis. Au. afarensis fossils have been found only in east Africa.
Cranial capacity to body mass was 1.2%, compared to 2.75% in humans today – roughly 1/3rd the brain size. Essentially, the anatomy of Au. afarensis appears apelike above the neck and humanlike below. Au. afarensis illustrates mosaic evolution: a variety of adaptive changes in separate places at disparate rates during different times.
There is no doubt that Au. afarensis walked upright: ranging the savannas and open woodlands, eating foraged fruits, seeds, sedges, and roots. Still, Au. afarensis retained a fondness for trees, where it spent much of its time.
Lucy is a famous Au. afarensis fossil find: a nearly complete female skeleton, 25 years old when she died 3.18 MYA.
Another find – a female juvenile dubbed Selam by its discoverers – suggests that Au. afarensis mixed walking with climbing. Selam’s shoulder blades and socket indicate adaptation for frequent tree-climbing. Selam presents an interpretive challenge. A terrestrial lifestyle while retaining some shoulder characteristics of earlier tree-climbing hominins is possible. Despite their similarities, Lucy and Selam may not have been the same species.
Au. afarensis had stone tools and practiced butchery. Ungulate bones found near Selam appear to have been cut by stone tools. But Au. afarensis lacked the manual dexterity to craft stone tools. Its hand proportions were more like gorillas than later hominins. While Au. afarensis may have been able to bring the tips of its fingers and thumb together, its thumbs were too short for the precision grip that later hominins had, enabling them as tool makers.
Like Ardipithecus ramidus, Au. afarensis were only moderately sexually dimorphic. Adult males stood ~1.6 meters and weighted 45 kg. Females were a bit smaller and less heavy. Males also typically displayed large crests at the top of their skulls; females did not.
Closely related hominin species shared the planet many times in the past few million years. ~ American paleontologist Bernard Wood
Au. afarensis had contemporaneous cousins. Besides closely related species in east Africa, one known relative was in North Africa; the other in the far south.
Lucy’s Northern Cousin
A 3.5-million-year-old foot fossil found in Ethiopia reveals a species like Ar. ramidus, at least from the ankle down. While Lucy walked about much of the time, this northern species had feet for climbing trees and grasping limbs. It would have walked awkwardly.
Lucy’s Southern Cousin
A nearly complete fossil skeleton dating to 3.67 MYA was found in South Africa’s Sterkfontein Caves in the mid-1990s. Similar partial skeletons had already been discovered at a nearby site in 1948. This small-footed hominoid with a flat face is known as Australopithecus prometheus.
Lucy’s Next-Door Neighbor
Current fossil evidence clearly shows that there were at least 2, if not 3, early human species living at the same time and in close geographic proximity. ~ Ethiopian paleoanthropologist Yohannes Haile-Selassie
3.4 MYA jaw fossils were found in Ethiopia in 2011, showing that Lucy had a nearby relative with a quite different diet. The species was dubbed Australopithecus deyiremeda.
Au. bahrelghazali is a controversial 1993 discovery by French paleontologist Michel Brunet. His find was a single specimen, dated 3.6 MYA. It has been the only australopithecine fossil found in central Africa. All others are from east Africa, some 2,500 km distant.
Au. bahrelghazali has similar dentistry to Au. afarensis, but is more like Sahelanthropus in the shape of its skull.
Stingy Brunet classified his find as a separate species, but refuses to let others examine the remains, contrary to the International Code of Zoological Nomenclature.
Au. africanus, which lived in southern Africa, dates to 3.3 MYA. Au. africanus was a significant step from Au. afarensis. Several anatomical features were significantly more like the Homo genus, including a larger cranium (400–625 cm2), gracile (slender) build, and more humanlike hands.
Au. africanus had a pelvis that was better built for walking than Au. afarensis. Yet Au. africanus retained the apelike curved fingers of tree climbers.
The diet of Au. africanus seems to have been seasonal, favoring fruit, though able to chew seeds and other food requiring mastication.
Sexual dimorphism showed in spinal adaptation of females to bear lumbar loads upright while pregnant.
Like chimpanzees, Au. africanus had patrilocal communities: in reaching sexual maturity, females emigrated to find mates.
A. garhi was a gracile species that lived 2.5 MYA. Large molars and pre-molars suggest a diet of tough, fibrous foods: perhaps tubers and other chewy vegetables. Au. garhi shaped stone tools. Only a single find of cranial fragments has been made of Au. garhi, in Ethiopia. Much mystery remains.
Au. sediba represents an amalgam. Fossils date to 2 MYA. Evident adaptations include better walking, even running, and a surprisingly modern hand capable of a precision grip. Vestiges of the past remained, albeit with a twist. Au. sediba had long arms, climbed trees, and walked upright but had a different gait from that of either chimps or humans. This suggests that several forms of bipedalism evolved among hominids.
While Au. sediba had a humanlike lower rib cage, its lower back was longer and more flexible than people today. Au. sediba brain size was on par with other australopiths (420–450 cm2), but the cranial shape presaged modern humans.
Au. sediba lived in woodlands, eating fruit, tree leaves, and bark. Au. sediba teeth were like Au. africanus; perhaps an instance of parallel evolution, as Au. sediba had no links to earlier hominins often regarded as Homo ancestors, notably Au. afarensis. Au. sediba may have simply been one of several different australopiths living at the time.
One of the main conclusions from discovering Kenyanthropus was that human evolution is a mosaic process, with different species showing unexpected combinations of anatomy. ~ English anthropologist Charles Lockwood
Kenyanthropus platyops (“flat-faced man of Kenya”) (3.5–3.2 MYA). The fossils of K. platyops were discovered in Lake Turkana, Kenya in 1999.
Kenyanthropus had a broad, flat face; striking with its high cheek bone and flat plane behind its nose bone. Its toe bone suggests that Kenyanthropus walked upright. Teeth were intermediate between ape and human characteristics. Its small ear hole is like that of a chimp. Its small brain fits with other early hominids.
Whether Kenyanthropus represents a new genus, a species of Australopithecus, or merely an Au. afarensis individual remains unsettled. It has been suggested that Kenyanthropus was one of numerous hominid species around that time, each adapted to a specific environment: that is, a product of adaptive radiation.
Paranthropus (“ape that lived alongside humans”) (2.7–1.2 MYA). The bipedal paranthrops probably descended from gracile australopiths but took a decided turn to the muscular.
Paranthropus had a robust anatomy, with massive jaws, sturdy bones, and strong muscles. Paranthrops lived off vegetation and possibly grubs, resorting to chewy fare only when starving.
Paranthrops were sexually dimorphic. While there was some size variation in the different paranthropic species, males were 1.2–1.4 meters, weighing ~68 kg, while females were 0.9–1.0 meters, 40–45 kilos. Males had a sagittal (cranial) crest (shades of Au. afarensis), but the smaller females did not. Like Au. africanus, Paranthropus were patrilocal.
P. aethiopicus lived 2.7–2.3 MYA; P. boisei: 2.6–1.3 MYA. P. robustus: 2.3–1.2 MYA.
Paranthrops’ brain size was that of a chimp today, ~40% that of a modern human; but paranthrops had significantly larger braincases than australopiths.
P. robustus had a hand adapted for a precision grip and tool use. It is unclear the extent to which paranthrops crafted or used tools. Paranthrops are thought to have preferred living in wooded areas, not grasslands.
Paranthropus were unrelated to the lineage that led to humans: hominids, not hominins. Paranthrops were on an evolutionary path that led to extinction.
The tough build of Paranthropus may have disguised a hominid poorly adapted to the demands of changing climate, at least compared to the upstart Homo genus that prevailed.