What do paleontologists who have studied the fossil remains of hominid feet from South Africa believe?

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Discovery of Early Hominins

 
The immediate ancestors of humans were members of the genus Australopithecus

.  The australopithecines

(or australopiths) were intermediate between apes and people.  Both australopithecines and humans are biologically similar enough to be classified as members of the same biological tribe--the Hominini

.  All people, past and present, along with the australopithecines are hominins

.  We share in common not only the fact that we evolved from the same ape ancestors in Africa but that both genera are habitually bipedal

, or two-footed, upright walkers.  By comparison, chimpanzees, bonobos, and gorillas are primarily quadrupedal

, or four-footed. 

Over the last decade, there have been a number of important fossil discoveries in Africa of what may be very early transitional ape/hominins, or proto-hominins.  These creatures lived just after the divergence from our common hominid ancestor with chimpanzees and bonobos, during the late Miocene and early Pliocene Epochs.  The fossils have been tentatively classified as members of three distinct genera--Sahelanthropus

, Orrorin

, and Ardipithecus

.  Sahelanthropus was the earliest, dating 7-6 million years ago.  Orrorin lived about 6 million years ago, while Ardipithecus remains have been dated to 5.8-4.4 million years ago.  At present, the vote is still out as to whether any of these three primates were in fact true hominins and if they were our ancestors.  The classification of Sahelanthropus has been the most in question.

The earliest australopithecines very likely did not evolve until 5 million years ago or shortly thereafter (during the beginning of the Pliocene Epoch) in East Africa.  The primate fossil record for this crucial transitional period leading to australopithecines is still scanty and somewhat confusing.  However, by about 4.2 million years ago, unquestionable australopithecines were present.  By 3 million years ago, they were common in both East and South Africa.  Some have been found dating to this period in North Central Africa also.  As the australopithecines evolved, they exploited more types of environments.  Their early proto-hominin ancestors had been predominantly tropical forest animals.  However, African forests were progressively giving way to sparse woodlands and dry grasslands, or savannas

.  The australopithecines took advantage of these new conditions.  In the more open environments, bipedalism would very likely have been an advantage.

By 2.5 million years ago, there were at least 2 evolutionary lines of hominins descended from the early australopithecines.  One line apparently was adapted primarily to the food resources in lake margin grassland environments and had an omnivorous diet that increasingly included meat.  Among them were our early human ancestors who started to make stone tools by this time.  The other line seems to have lived more in mixed grassland and woodland environments, like the earlier australopithecines, and was primarily vegetarian.  This second, more conservative line of early hominins died out by 1 million years ago or shortly before then.  It is likely that all of the early hominins, including humans, supplemented their diets with protein and fat rich termites and ants just as some chimpanzees do today.

Major early hominin sites

History of Discovery

In his 1871 book entitled The Descent of Man, and Selection in Relation to Sex, Charles Darwin speculated that fossils of the earliest humans and their immediate progenitors ultimately would be found somewhere in Africa.  He based this on the fact that the natural range of our nearest living relatives, chimpanzees and gorillas, is limited to Africa.  He concluded that we ultimately must have shared a now extinct common ancestor with those apes in Africa.  This view was mostly rejected by the scientific world of the time.  Before the 1920's, knowledge of our fossil ancestors only went back to the Neandertals

in Europe and some presumably earlier human-like forms from Java, in Southeast Asia.  Few researchers were willing to estimate the time period of the earliest hominins at much more than 100,000 years, and there was no inkling of anything older from Africa.  In addition, there was a bias among the predominantly European paleoanthropologists against accepting early Africans as the ancestors of all humanity.

Raymond Dart   (1893-1989)

"Taung child" reconstruction   (Australopithecus africanus)

In 1924, Raymond Dart, an Australian anatomy professor at the University of Witerwatersrand in Johannesburg, South Africa, obtained a fossil skull that had been blasted out of a nearby limestone quarry at Taung

.  It took him 73 days to chisel the skull free from its surrounding stone matrix and 4 years of spare time to free the jaw and the fossilized brain.  However, long before then, Dart recognized the importance of this find.  In 1925 he named it Australopithecus africanus

(literally "southern ape from Africa").  Because of its small size, he called it the "Taung baby."  In fact, its teeth indicate that it was a 3-4 year old child.  Despite its relatively small brain, he concluded that this species was intermediate between apes and humans.  He based this mainly on the shape and position of the base of the brain cast.  It indicated that the foramen magnum

, or hole in the skull through which the spinal cord passes, pointed downward and was nearly at the central balance point of the skull.  This meant that the Taung child must have been bipedal.  In addition, the canine

teeth were relatively short.  In both of these traits, the Taung child was much more like a human than an ape.  Most paleoanthropologists in the 1920's rejected Dart's claims that Australopithecus africanus was intermediate between apes and humans in favor of the view that it was just an ape.  Dart's claims were not widely accepted until the late 1940's.

Robert Broom (1866-1951)

Following Dart's discovery, several other caves were investigated in South Africa.  Most of the work was done by Robert Broom from 1936 through the 1940's.  Broom was a medical doctor and an enthusiastic amateur paleontologist from Scotland.  In 1903, he was appointed professor of geology at the University of Stellenbosch in South Africa and became internationally respected for his studies of early mammal-like reptiles.  His insistence on the correctness of the theory of evolution led to his dismissal from this conservative religious university in 1910.  Consequently, he returned to being a medical doctor in a rural town in South Africa but continued paleontological research in his spare time.  In 1934, at the age of 68, he retired from his medical practice and joined the staff of the Transvaal Museum in Pretoria as a paleoanthropologist.  The rest of his life was spent searching for early hominin fossils.  

Robert Broom's most important discoveries were made in the Sterkfontein valley of South Africa.  It was there in 1936 that he found the first known adult Australopithecus africanus while excavating in Sterkfontein

cave.  In 1938, he discovered more fossil remains of africanus and other early hominins in Kromdraai

cave.  Some of these fossils were larger boned and more muscular with powerful jaws.  Broom named them Paranthropus robustus

(Paranthropus means "parallel to man").  Significantly, these robust hominins also differed in having a sagittal crest

, or ridge of bone extending from front to back, along the midline of the top of the skull.  A sagittal crest serves as an anchor attachment for exceptionally large, strong jaw muscles.  This skeletal feature is also present in large apes but not in africanus or humans.

Australopithecus africanus				Paranthropus robustus

NOTE:  Some paleoanthropologists lump Paranthropus robustus and other paranthropoids into the genus Australopithecus.  They consider them to be a physically robust subgroup of australopithecines.

In 1948, Robert Broom found more paranthropoid fossils at Swartkrans

cave in South Africa.  Following that excavation, he dedicated the rest of his life to writing everything known about all of the early hominins.  He completed this compendium work in 1951.  He was 85 years old and ill.  As he finally finished his writing, he reportedly said "now it is done and so am I."  He died a few minutes later.

Leopard canines fit  punctures in hominin  skull from Swartkrans

Between 1965 and 1983, Swartkrans cave was carefully reinvestigated by another South African paleoanthropologist, C. K. Brain, using more thorough field and laboratory techniques than had been used by Robert Broom a generation earlier.  Many thousands of bone fragments, including the remains of 130 individual hominins, were recovered by Brain.  These bones were from australopithecines and paranthropoids as well as early members of our genus, Homo.  Because many of the bones had chewing marks and at least one of the skulls had peculiar depressions reminiscent of punctures made by the canine teeth of a leopard, Brain hypothesized that some of the Swartkrans hominins had been eaten by these big cats.  The early hominin fossil-bearing strata in the cave also contained 195 stones that were from locations distant from the cave.  Brain believed that 30 of them may have been used as tools or weapons.  In any case, the presence of these stones suggests that not all of the early hominins in the cave were there as a result of being the victims of carnivores.

Unfortunately, most of the South African sites where early hominin fossils have been found are not easily dated because they lack association with volcanic deposits that would readily allow radiometric dating.  That is not the case with most of the early hominin sites in East Africa.

The oldest fossil hominins have been recovered from sites in East Africa, especially in the Great Rift Valley.  One of the most important sites there is Olduvai Gorge

.  It is an approximately 30 mile (48 km.) long, eroded canyon complex cutting into the Serengeti

Plain in Northern Tanzania.  It is only about 295 feet (90 m.) deep, but its neatly stratified layers of dirt and rock interspersed with easily datable volcanic ash and lava layers cover the last 2.1 million years of geological and evolutionary history.  The remains of many australopithecines, paranthropoids, and early humans have been found at Olduvai.  When these ancient hominins lived there, it was a lake margin grassland area that had abundant plant food and meat sources that could be exploited by scavenging. 

Mary and Louis Leakey with the "Zinjanthropus boisei" palate and a modern human skull in 1959

Zinjanthropus boisei (Paranthropus boisei)

Early hominin fossils from Olduvai Gorge are known mostly as a result of the many expeditions of Louis and Mary Leakey

.  Louis began searching there in 1931, and his second wife Mary joined him in 1935.  However, it was not until 1959 that they found their first early hominin fossil.  Louis gave it a new genus and species designation, Zinjanthropus boisei

(literally "East African man").  Subsequently, it was recognized to be only a super robust paranthropoid.  It is now generally referred to as Paranthropus boisei

.  Using the then new potassium-argon dating method, the fossil was determined to be 1.75 � .25 million years old.  This was a startlingly early date when it was made public in 1959.  Louis Leakey and Zinjanthropus instantly became international media stars, and both of their pictures were on the front page of newspapers around the world.  Louis was also the focus of several television documentary programs.  In the years after his death in 1972, Mary became well known as a paleoanthropologist in her own right.

NOTE:  Louis Leakey gave his Zinjanthropus find the species name boisei in honor of Charles Boise, a wealthy American who funded fieldwork by the Leakeys.

In 1974, a team of paleoanthropologists, under the direction of an American, Donald Johanson, found an even more ancient species of australopithecine at the Hadar

site in the Afar

Desert region of Northern Ethiopia.  It was a 40% complete skeleton of an adult female whom they named Lucy.  She had been only 3 feet 3 inches (1 m.) tall with a slender body weighing only about 60 pounds (27 kg.).  She lived 3.2-3.18 million years ago.  Johanson concluded that Lucy was from a different species than had been previously discovered.   He classified her as an Australopithecus afarensis

(named for the Afar region).  Many other specimens of this species and later ones were found in Ethiopia since 1974, but none is as complete as Lucy. 

"Lucy" skeleton (Australopithecus afarensis)

Finding Lucy--how she was discovered         This link takes you to a video at an external website.  To return here, you must         click the "back" button on your browser program.     (length = 4 mins, 33 secs)   Becoming a Fossil--how Lucy was preserved         This link takes you to a video at an external website.  To return here, you must         click the "back" button on your browser program.     (length = 2 mins, 34 secs)

Early hominin footprints at Laetoli, Tanzania

About 30 miles south of Olduvai Gorge in Northern Tanzania is the Laetoli

site.  It was investigated in the late 1930's by Louis and Mary Leakey, but no fossil hominins were found at that time.  Mary Leakey returned to Laetoli with Tim White, an American paleoanthropologist in 1978.  They found bones of what were likely Australopithecus afarensis dating 3.7-3.5 million years ago (several hundred thousand years older than Lucy).  They also found 59 footprints of bipedal hominins (presumably afarensis) in a now hardened volcanic ash layer.  These individuals walked in two close parallel tracks across volcanic dust at least 3.5 million years ago.  The footprints look almost like those of modern humans.  They are narrow with a low arch, and they clearly show that the big toe was in line with the others.  These are all traits of humans but not of apes.  Based on the characteristics of the footprints, Mary Leakey concluded that their makers were adults who were 4 feet 9 inches and 4 feet 1 inch tall and that they walked parallel to each other in a strolling fashion with relatively short strides.  In addition, there are a child's footprints within those of the larger adult.

Laetoli Footprints--how they were formed and preserved         This link takes you to a video at an external website.  To return here, you must         click the "back" button on your browser program.     (length = 3 mins, 20 secs)

The "Black Skull" (Paranthropus aethiopicus)

In northern Kenya and southern Ethiopia, there is an arid region around Lake Turkana

in the Great Rift Valley that has exposed geological deposits dating to at least 4.3 million years ago.  Richard Leakey, the son of Mary and Louis Leakey, began looking for hominin fossils there in the late 1960's.  During the 1970's, his team of field researchers from the National Museum of Kenya made a number of important finds, including fossils of early humans who will be described in the next tutorial of this series. 

While working on the western side of Lake Turkana in 1985, an American paleoanthropologist named Alan Walker made an important discovery.  This was a nearly complete paranthropoid skull with an unusually large sagittal crest and some features reminiscent of the more ancient Australopithecus anamensis (described below).  Manganese in the soil deposit where it was located stained it black.  As a result, this unusual fossil has become known as the "black skull."  It has been classified as Paranthropus aethiopicus (named after Ethiopia).  Since it dates to 2.5 million years ago, it is a prime candidate for being the earliest paranthropoid species.

In 1995, Meave Leakey, the wife of Richard Leakey, began discovering bones of a very early australopithecine species at several sites southwest of Lake Turkana.  She named it Australopithecus anamensis

("anam" is "lake" in the Turkana language).  The dentition of this hominin seems to be transitional between apes and later australopithecines.  This fits with the 4.2-3.9 million year dates for the volcanic ash associated with the anamensis fossils.  The shapes of the arm and leg bones of this species indicate that it was bipedal.  Anamensis is currently the earliest known australopithecine species.  Bones from at least 8 more anamensis skeletons have been found in Ethiopia.

In 1996, Berhane Asfaw, an Ethiopian researcher, and Tim White found a 2.5 million year old hominin fossil in the Middle Awash Valley of Ethiopia that remains problematical.  The skull characteristics are similar in some ways to Australopithecus afarensis, but it lived several hundred thousand years after that species had presumably ended.  Asfaw named his new hominin, Australopithecus garhi

("garhi" is "surprise" in one of the languages of the Afar Desert region).  Whether or not garhi was a late afarensis, a variant of africanus, or a distinct species is not clear.  Associated with the remains of garhi were animal bones with what appear to be cut marks made by simple stone tools.   If they were using such tools, this is remarkable because only early humans have heretofore been associated with stone tool making.

In 2001, Meave Leakey announced the discovery of a 3.5-3.2 million year old hominin skull from the west side of Lake Turkana.  She suggested that this fossil may displace Lucy (Australopithecus afarensis) as the progenitor of humans.  Meave named it Kenyanthropus platyops ("flat-faced man of Kenya").  This hominin lived during the same time period as Lucy.  However, it had a comparatively large, flat face and smaller teeth.  The latter characteristic suggests that Kenyanthropus regularly ate softer foods than did Lucy.  However, it is not yet clear where this new discovery fits within our evolution.  Some paleoanthropologists have suggested that it is only a variant of Australopithecus afarensis.

Recent Discoveries

In 2006, Tim White, now a professor at the  University of California, Berkeley, announced the discovery of bones from at least 8 Australopithecus anamensis individuals dating to 4.1 million years ago in what had been a woodland environment in the Awash Valley of Ethiopia.  In 2009, White and his team of researchers announced the discovery of a 4.4 million year old ape/transitional species named Ardipithecus ramidus that also lived in woodland environments of the Awash Valley.  White believes that this very early species was the direct ancestors of Australopithecus afarensis.

The Analysis of Ardipithecus ramidus--one of the earliest known hominids         This link takes you to a video at an external website.  To return here, you        must click the "back" button on your browser program.         (length = 10 mins)

In 2010, Lee Berger of the University of the Witwatersrand in Johannesburg, South Africa announced his discovery of two partial skeletons of what may be a new australopithecine species that lived 1.977 million years ago in South Africa.  He named it Australopithecus sediba ("sediba" means "fountain" or "wellspring" in the seSotho language of South Africa).  Berger and his colleagues suggest that this new species may be descended from Australopithecus africanus and could be one of the last links in the evolutionary line between the australopithecines and our genus Homo.

Australopithecus sediba: New Hominin Species Found--Stephen Churchill from Duke University         describes key fossils and their potential importance.  This link takes you to a video at an         external website.  To return here, you must click the "back" button on your browser program.         (length = 3 mins, 4 secs)

NOTE:  Our understanding of early hominins was led astray at the beginning of the 20th century as a result of the discovery by Charles Dawson in 1912 of a fossil skull in England that became known as the Piltdown man.  It had a large brain case similar to modern humans but an ape-like jaw.  This fit with the popular but incorrect assumption that our early ancestors would have ape-like bodies and human-like brains.  The discovery of australopithecines in South Africa beginning in 1924 showed that the early hominins were actually just the reverse―they had almost human-like bodies below the neck but brains that were very little changed in size from those of apes.  It was not until the early 1950's that the Piltdown man skull was exposed for what it really was, a clever fraud.  This realization came as a result of close examination by independent researchers and fluorine analysis dating.

Copyright � 1999-2012 by Dennis O'Neil. All rights reserved.
illustration credits

Page 2

Analysis of Early Hominins

The bones of more than 500 early hominins have been found.  From them, we have gained a broad understanding of these related species using an array of new technological aids.

New Technology for Old Fossils--members of the Human Origins Program team of the Smithsonian         Institution describe how they use cutting-edge technology in their scientific investigations.         This link takes you to a video at an external website.  To return here, you must click the "back" button         on your browser program.         (length = 2 mins, 23 secs)

It is now understood that while there were considerable anatomical differences between the early hominins, they also shared a number of important traits.  By 3 million years ago, most of them probably were nearly as efficient at bipedal locomotion as humans.  Like people, but unlike apes, the bones of their pelvis, or hip region, were shortened from top to bottom and bowl-shaped (shown below).  This made the pelvis more stable for weight support when standing upright or moving bipedally.  The longer ape pelvis is adapted for quadrupedal locomotion.  Early hominin leg and foot bones were also much more similar to ours than to those of apes.  This is consistent with the likelihood of early hominin bipedalism.

Comparison of Pelvis and Foot Bones

Bipedal locomotion may have been an adaptation to living in a mixed woodland and grassland environment.  It has been suggested that bipedalism was selected for because it made it easier to see long distances when moving over areas covered with tall grasses.  This would have been a useful advantage in scavenging for food and watching for big cats and other predators in open environments.  An upright posture also potentially helps to dissipate excess body heat and reduces the absorption of heat from the sun because less skin has a direct exposure to ultra violet radiation during the hottest times of the day.  There is evidence suggesting that bipedal animals usually can walk greater distances because less energy is expended with their longer strides.  This would be useful for scavenging for food throughout vast areas.  However, the legs of bipedal animals need to be sturdy enough to support at least 2.5 times their body weight while running.  Over many generations, early hominin legs grew longer and much stronger than their arms.  Their feet became longer and developed arches for more efficient support of their bodies.  In addition, their hands became more adept at carrying and manipulating objects such as tools and food.  It also made it easier to hold babies and to tend to their needs.  These adaptations to walking bipedally on the ground made it progressively more difficult to climb and travel through the canopies of trees.  However, they obviously provided many other natural selection advantages.

Walking Tall--a comparison between human and chimpanzee skeletons         This link takes you to a video at an external website.  To return here, you must         click the "back" button on your browser program.         (length = 56 secs)

Baby Steps: Learning to Walk, The Hominid Way--the evolution of bipedalism        among our hominid ancestors.  This link takes you to an audio file at an        external website.  To return here, you must click the "back" button on your         browser program.         (length = 7 mins, 46 secs)

Chimpanzees and all of the other apes have longer arms than legs and lack arches on the bottoms of their feet.  In addition, their big toes are divergent from their other toes much like human thumbs.

While the late australopithecines were similar to humans anatomically below the neck, their heads were significantly different from ours in several key features.  Their adult brain size was about 1/3 that of people today.  As a result, the widest part of the skull of these early hominins was below the brain case.  For modern humans, it usually is in the temple region.  Early hominin faces were large relative to the size of their brain cases.  They had comparatively big molar teeth with thick enamel.  By comparison, their front teeth were small.  They had large jaws, and powerful jaw muscles.  The size and shape of these muscles is indicated by flaring zygomatic arches

, or cheek bones, behind which the major jaw muscles pass and the presence of a sagittal crest

, which is a jaw muscle attachment ridge of bone on top of the skull in the robust species (paranthropoids).  In modern humans, the jaw muscles are much smaller and attach onto the skull in the temple region.  From the side view, early hominin faces were concave or dish-shaped and projecting forward at the bottom due to their relatively small brain cases and huge teeth and jaws.  In contrast, our teeth and jaws are relatively small, and our faces are nearly vertical.

Paranthropus boisei	Modern human

NOTE:  When the lower portion of the face markedly projects forward (as in the case of the early hominins), it is known as prognathism.

Australopithecine and other early hominin fossils have been found only in Africa.  The majority of them were discovered in East and South Africa.  However, some also were found in Chad, which is located in North Central Africa.  Current evidence indicates that there were as many as 12 species of early hominins between 6 and 1.5 million years ago, but they did not all live at the same time.  The following species are the most widely accepted ones:

Paranthropus aethiopicus (or Australopithecus aethiopicus)

1.	Australopithecus anamensis
2.	Australopithecus afarensis
3.	Australopithecus africanus
4.
5.	Paranthropus boisei (or Australopithecus boisei)
6.	Paranthropus robustus (or Australopithecus robustus)

The fossil record of early hominins is being added to by new important discoveries almost every year.  As a result, it is not yet clear how many species of them actually existed nor is it certain what their evolutionary relationship was to each other.  However, the broad outlines of this complex evolutionary history are already known and are summarized here.  To see a more complete listing of proposed species of early hominins and their immediate ancestors, select the button below.  It would be helpful to have a printout of this table in order to understand the discussion of the early hominins that follows.

Table of Early Hominins and Their Immediate Ancestors

Australopithecine Species

Australopithecus anamensis may have been the earliest australopithecine species.  They lived about 4.2-3.9 million years ago in East Africa.  Unfortunately, little is known about them due to the scarcity of their fossils and the fact that the ones that have been found are highly fragmentary.  This species apparently was descended from Ardipithecus ramidus, which lived around 4.4 million years ago, or an even earlier ape/hominin transitional species near the beginning of the Pliocene Epoch.  Anamensis was bipedal but may still have been an efficient tree climber.  The shapes of the arm and leg bones indicate that it was bipedal.  The canine teeth are relatively large compared to later australopithecines and humans.  The alignment of teeth in the jaw is somewhat rectangular, reminiscent of apes, rather than like the modern human parabolic dental arch (like the McDonald's golden arches sign).  Anamensis remains have been found in what had been woodlands around lakes.  Their diets were apparently mainly vegetarian with an emphasis on fruits and nuts.

Australopithecus afarensis lived about 3.7-3.0 million years ago in East Africa.  Skeletally, they were still somewhat transitional from earlier ape species.  This can be seen in their legs which were relatively shorter than those of the later australopithecines and humans.  Afarensis also had slender curved fingers reminiscent of chimpanzees.  Because of these anatomical characteristics, it has been suggested that they were less efficient bipeds and more efficient tree climbers than the later australopithecines.  Afarensis canine teeth were relatively large and pointed, reminiscent of apes.  They projected somewhat beyond their other teeth but not as much as in chimpanzees.  Some of the male afarensis had small sagittal crests.

Australopithecus afarensis (Lucy)		Australopithecus afarensis (reconstructed appearance)

Kenyanthropus platyops (reconstructed appearance)

Tim White and some other paleoanthropologists believe that there was considerable physical variation within the species Australopithecus afarensis.  They suggest that the recently discovered fossils classified as Kenyanthropus platyops (3.5-3.2 million years ago) was a variant form of afarensis but with somewhat smaller teeth.  White discounts the flattened face of platyops as being due to the deformation of the bones by ground pressure after death.  Its discoverer, Meave Leakey, disagrees.  She believes that platyops was a separate species and that it was more likely to have been the progenitor of humans.  Additional hominin fossils from the crucial time period of 4-3 million years ago must be discovered to conclusively determine the place of platyops in our evolution.

Australopithecus africanus

Australopithecus africanus lived about 3.3-2.5 million years ago in South and East Africa.  Skeletally, they were less ape-like than earlier species of australopithecines but were still usually small and light in frame like afarensis.  However, the teeth of africanus were in some ways more like humans than like afarensis.  Specifically, the front teeth of africanus were relatively large like ours and their canine teeth did not project beyond the others.  Microscopic wear patterns on africanus teeth suggest a diet consisting of relatively soft foods, which very likely included some meat along with plants.  This does not necessarily imply efficient hunting skills.  More likely, they obtained meat by scavenging what remained on the abandoned corpses of large animals killed by lions and other predators.  It is possible that they also did some hunting of small animals in much the same inefficient manner of chimpanzees today.  They probably ate insects and eggs as well.

The classification of Australopithecus garhi is still very problematical.  This Ethiopian fossil has been dated to 2.5 million years ago, which makes it contemporaneous with late africanus.  Largely for that reason, some paleoanthropologists have suggested that garhi is a variant of africanus.  However, several features of the head of garhi look more like a holdover from the older afarensis species.  On the other hand, the relative lengths of the arms and legs of garhi are more reminiscent of the first humans.  The discovery of butchered animal bones with garhi suggests that their diet included at least some meat, as was the case with africanus.

Paranthropoid Species

The australopithecines have been referred to collectively as gracile

species (literally "gracefully slender") of early hominins.  Most of them were relatively small, slender, and delicate boned compared to the somewhat more muscular, robust species (paranthropoids) that mostly came later.  However, this is not always a reliable descriptive distinction because the range of variation in physical appearance of the two groups of species overlaps.  Subsequently, some individual graciles were bigger than some of the robust ones.  However, the robust species shared some characteristics of their heads that dramatically show that they had diverged from the evolutionary line that would become humans.  They had larger faces and jaws accompanied by pronounced sagittal crests (in the case of males).  They also had much larger back teeth (premolars and molars) and smaller front ones (incisors) compared to gracile australopithecines and early humans who were alive at the same time.

Australopithecus    (gracile body)	Paranthropus (robust body)		Paranthropus      teeth (upper)	human teeth (lower)

Little is known about Paranthropus aethiopicus

(the "black skull") other than it apparently was one of the earliest robust species--it lived about 2.5 million years ago.  So far, this species has been found only in East Africa.  Since it had a smaller brain than the other robust species and it was early, aethiopicus is thought to be a transitional form from one of the gracile species that came before.  It had an unusually large sagittal crest (shown below). 

Paranthropus robustus was a South African robust species that lived about 2.0-1.4 million years ago.  They had strong jaws and very large molar and premolar teeth with thick enamel.  Males also had pronounced sagittal crests, though not as large as the species listed next.

Paranthropus boisei was a super-robust East African species that lived about 2.0-1.4 million years ago.  They tended to be more massive and beefy-looking even than Paranthropus robustus.  Male boisei were especially muscular.  Like their South African cousins, robustus, they had prominent sagittal crests and very large grinding teeth with thick enamel.  These teeth would have been capable of cracking hard nuts and dry seeds.  However, such food items may not have been important in their diet.  Microscopic analysis of dental wear patterns and carbon isotope analysis of teeth indicate that what boisei predominantly ate was soft foods such as grasses, leaves, roots, and possibly even meat.

Paranthropus aethiopicus	Paranthropus  robustus	Paranthropus boisei

Early Hominin Body Size

The early hominins were significantly smaller on average than modern humans.  Adult male australopithecines were usually only about 4.3-4.9 feet tall and weighed around 88-108 pounds.  Females were much smaller and less muscular.  They were usually 3.4-4.1 feet tall and weighed only 64-75 pounds.  This is greater sexual dimorphism

than is found in human populations today.  In some australopithecine species, sexual dimorphism may have been nearly as great as among the great apes.  Female gorillas weigh about 61% that of males, while modern human females are about 83% the weight of males.

	AVERAGE WEIGHT	AVERAGE STATURE
SPECIES	males	females	females as   % of males	males	females	females as   % of males
Australopithecus afarensis	92 lbs (42 kg)	64 lbs (29 kg)	64%	4 ft 11 in (151 cm)	3 ft 5 in (105 cm)	70%
Australopithecus africanus	90 lbs (41 kg)	66 lbs (30 kg)	73%	4 ft 6 in (138 cm)	3 ft 9 in (115 cm)	83%
Paranthropus robustus	119 lbs (54 kg)	88 lbs (40 kg)	74%	3 ft 9 in (114 cm)	3 ft 3 in (99 cm)	87%
Paranthropus boisei	108 lbs (49 kg)	75 lbs (34 kg)	69%	5 ft 4 in (137 cm)	4 ft 1 in (124 cm)	91%
earliest humans      (Homo habilis)	114 lbs (52 kg)	70 lbs (32 kg)	61%	5 ft 2 in (157 cm)	4 ft 1 in (125 cm)	79%
modern humans 1      (Homo sapiens)	144 lbs (65 kg)	119 lbs (54 kg)	83%	5 ft 9 in (175 cm)	5 ft 3 in (161 cm)	92%
1  The relatively low weight and height of modern humans shown here is a rough average of all people     around the globe.  Some populations are significantly bigger (e.g., many Europeans and Africans). (Source: H. M. McHenry, "How Big Were Early Hominids?", Evolutionary Anthropology 1 [1992] p. 18; and "What Does It Mean To Be Human? Human Evolution Evidence" [2010}, Smithsonian National Museum of Natural History)

Possible Evolutionary Links

There has been a gap in the fossil hominin record for the crucial period before 4.2 million years ago when Australopithecus anamensis appeared.  New discoveries are now beginning to fill in the missing picture of evolution leading to the australopithecines at that early time.  Beginning in 1992, Tim White and several of his Ethiopian colleagues found fossils of what may be the immediate ancestor of the australopithecines at the Aramis site in the Middle Awash region of Northern Ethiopia.  The teeth of these very early fossils seem to have been transitional between apes and Australopithecus anamensis.  Among the living apes, they were most similar to chimpanzees, however, they were not apes as we usually think of them today.  These Aramis fossils date to about 4.4 million years ago and may represent the first stage in the evolution of bipedalism.  Because of their primitiveness, White has given them a new genus and species designation (Ardipithecus ramidus

, nicknamed "Ardi") rather than include them with australopithecines.

Based on the time frame, body shape, and dentition similarities, it is reasonable to conclude that some of the early hominin species were ancestors of our genus Homo.  Most likely, some of the australopithecines (shown as red in the diagram below)  were in our line of evolution, but the later paranthropoids (blue below) were not.  The first humans (Homo habilis

) were contemporaries of the paranthropoids.  As a result, they could not be our ancestors.  However, it is likely that Australopithecus afarensis and Australopithecus africanus were in our evolutionary line.   Australopithecus garhi and/or Australopitheus sediba may also have been our ancestors, though more evidence is needed to settle this question.

We have not yet been able to extract DNA from the bones of any australopithecine for comparison with modern human DNA.  When we can do this, it is almost certain that we will discover many of their genes still in us today.

Beginning around 2.5 million years ago or a bit earlier, there was a major forking in the evolutionary path of hominins.  The australopithecines diverged into at least two very different evolutionary directions.  One led to the paranthropoids and a genetic dead-end by about 1.4 million years ago.  The other led to the first humans.  It is likely that these diverging evolutionary paths were the result of exploiting different environmental opportunities.  Coinciding with this hominin divergence was a shift in the global climate to progressively cooler conditions and frequently fluctuating environments.  In East and South Africa, where most of the early hominins apparently lived, dry grasslands expanded at the expense of woodlands and forests.  It has been suggested that the adaptive radiation that led to humans and paranthropoids is connected with this change in the environment.  The early human line succeeded by learning how to exploit new kinds of habitats for food.  It is likely that climate instability selected for their adaptability.

Becoming Human: Part 1----NOVA episode about the complex evolutionary picture of early hominins         that is emerging from the fossil record.  This link takes you to a video at an external website.  To return         here, you must click the "back" button on your browser program.     (length = 51 mins, 57 secs)

NEWS:  John Novembre et.al. reported in the October 1, 2007 issue of Nature Genetics that human saliva has significantly more of the enzyme amylase compared to chimpanzees.  Amylase breaks down starches into glucose which can be readily used by the cells of the body.  With more amylase, humans get more useable calories from starchy vegetable foods such as tubers, corms, and bulbs.  The authors suggest that this would have been a distinct advantage for early humans because these foods are readily available.  They believe that natural selection favored additional copies of the gene responsible for amylase production (AMY1) in our early hominin ancestors but not in apes.

Copyright � 1999-2012 by Dennis O'Neil. All rights reserved.
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