Analysis of Early Hominids

[Brooke]

Analysis of Early Hominids


The bones of more than 500 early hominids have been found.  From them, we have gained a broad understanding of these related species.  While there were considerable anatomical differences between them, 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 or moving bipedally.  The longer ape pelvis is adapted for quadrupedal locomotion.  Early hominid leg and foot bones were also much more similar to ours than to those of apes.  This is consistent with the likelihood of early hominid bipedalism.

Comparison of Pelvis and Foot Bones


Bipedal locomotion may have been an adaptation to living in relatively open grassland environments.  It is likely that bipedalism was selected for because it made it easier to see long distances.  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 since less surface area has a direct exposure.  There is evidence suggesting that bipedal animals usually can walk and run greater distances because less energy is expended with their longer strides.  This would be useful for scavenging 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 hominid legs became longer and much stronger than their arms.  Their feet developed arches for more efficient support of their bodies.  In addition, their hands became more adept at manipulating objects such as tools and food.

[Brooke]

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 hominids was below the brain case.  For modern humans, it usually is in the temple region.  Early hominid faces were large relative to the size of their brain cases.  They had comparatively big teeth with thick enamel, large jaws, and powerful jaw muscles. The size and shape of these muscles is indicated by flaring zygomatic arches  and the presence of a sagittal crest , which was a jaw muscle attachment area in the robust species.  In modern humans, the jaw muscles are much smaller and attach onto the skull in the temple region.  From the side view, early hominid faces were concave or dish-shaped and projecting forward at the bottom due to their large teeth and jaws.  In contrast, our jaws are relatively small and our faces are nearly vertical.

 
 
       Australopithecus boisei              Modern human

Early hominid fossils have been found only in Africa.  The majority of them were discovered in East and South Africa.  However, some also were found recently in Chad, which is located in North Central Africa.  Current evidence indicates that there were at least 6 species of early hominids, though they did not all live at the same time.

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

The fossil record of early hominids 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 hominids 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 hominids that follows.

  Table of Early Hominids and Their Immediate Ancestors   

[Brooke]

Early 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 they are highly fragmentary.  However, they apparently were descended from Ardipithecus ramidus or an even earlier ape/hominid transitional species near the beginning of the Pliocene Epoch.  Anamensis probably was only partially bipedal and still an efficient tree climber.

 
Australopithecus afarensis
(Lucy)
 
Australopithecus afarensis lived about 4.0-2.9 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.

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 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.  Likewise, Australopithecus bahrelghazali may be a regional variation of afarensis.  If this turns out to be the case, then we would have to conclude that afarensis lived in North Central Africa as well as East Africa.


 
Australopithecus africanus
Australopithecus africanus lived about 3.3-2.3 million years ago in both East and South 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 meat along with plants.  This does not necessarily imply efficient hunting skills.  More likely, they obtained much of their meat by scavenging what remained on the corpses of 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.

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.

[Brooke]

Late Australopithecine Species

The early australopithecines have been referred to collectively as gracile  species (literally "gracefully slender").  Most of them were relatively small, slender, and delicate boned compared to the somewhat more muscular, robust species of australopithecines 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.  The late australopithecines, which were all robust species, had larger jaws accompanied by pronounced sagittal crests in the case of males.  The robust species of australopithecines all lived after 2.5 million years ago. 


 
     gracile form     robust form       

Little is known about Australopithecus 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).

Australopithecus robustus was a South African robust species that lived about 2.0-1.5 million years ago.  Their teeth appear to have been adapted to eating tough, fibrous vegetable foods.  This is indicated by their strong jaws and very large molar and premolar teeth with thick enamel.  Microscopic wear patterns on their teeth indicate that they regularly ate foods such as hard nuts and seeds.  Males also had pronounced sagittal crests, though not as large as the next species listed here.

Australopithecus boisei was a super-robust East African species that lived about 2.0-1.2 million years ago.  They tended to be more massive and beefy-looking even than Australopithecus robustus.  While they were only a few inches taller, they averaged 20 pounds heavier.  Male boisei were especially muscular.  Like their South African cousins, robustus, they had prominent sagittal crests and very large grinding teeth with thick enamel.  This suggests a similar diet of hard vegetable foods.

       

Australopithecus aethiopicus   
(the "black skull")

Australopithecus robustus

Australopithecus boisei   
 (formerly known as   
 Zinjanthropus)

[Brooke]

Early Hominid Body Size

The early hominids 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.  Apparently, 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. 


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 BODY WEIGHT STATURE
 
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SPECIES males     females     females as 
 % of males   males     females     females as 
 % of males

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  Australopithecus afarensis 1   99 lbs
(45 kg) 64 lbs
(29 kg) 64%  59 in
(151 cm) 41 in
(105 cm) 70%
  Australopithecus africanus    90 lbs
(41 kg) 66 lbs
(30 kg) 73% 54 in
(138 cm) 45 in
(115 cm) 83%
  Australopithecus robustus 88 lbs
(40 kg) 70 lbs
(32 kg) 80% 52 in
(132 cm) 43 in
(110 cm) 83%
  Australopithecus boisei 108 lbs
(49 kg) 75 lbs
(34 kg) 69% 54 in
(137 cm) 49 in
(124 cm) 91%
  earliest humans
     (Homo habilis)   114 lbs
(52 kg) 70 lbs
(32 kg) 61% 62 in
(157 cm) 49 in
(125 cm) 79%
  modern humans 2
     (Homo sapiens) 144 lbs
(65 kg) 119 lbs
(54 kg) 83% 69 in
(175 cm) 63 in
(161 cm) 92%

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Source: H. M. McHenry, "How Big Were Early Hominids?", Evolutionary Anthropology 1 [1992] p. 18.
 
1  Afarensis may have been somewhat less sexually dimorphic than indicated by McHenry's data
    presented here (Phillip Reno et al., "Sexual Dimorphism in Australopithecus afarensis was Similar
    to that of Modern Humans, Proceedings of the National Academy of Sciences [2003] pp. 9404-9409).
2  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., Europeans and Africans).


Possible Evolutionary Links

There has been a gap in the fossil hominid 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 hominids 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.  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 ) rather than include them with australopithecines.
 

Based on body shape and dentition similarities, it is reasonable to conclude that some of the early hominid species were ancestors of our genus Homo.  Most likely, some of the early australopithecines (shown in red below)  were in our line of evolution, but the later robust ones (blue below) were not.  The first humans (Homo habilis ) were contemporaries of the late australopithecines.  As a result, they could not be our ancestors.  However, it is likely that Australopithecus afarensis and possibly even early Australopithecus africanus or Australopithecus garhi were in our evolutionary line.



Beginning around 2.5 million years ago or a bit earlier, there was a major forking in the evolutionary path of hominids.  The australopithecines diverged into at least two very different evolutionary directions.  One led to the robust australopithecines and a genetic dead-end by about 1.2 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 hominid divergence was a shift in the global climate to cooler conditions.  In East and South Africa, where most of the early hominids 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 robust australopithecines is connected with this change in the environment.

 

  Search for the First Human--Donald Johanson talks about the complex
        evolutionary picture of early hominids 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 = 2 mins, 10 secs)
 
        Note: the Orrorin referred to in the video is Orrorin tugenensis, a possible
        ancestor of the australopithecines that lived about 6 million years ago.   

 


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NOTE:  This is not the only possible model of early hominid evolutionary links that has been suggested in recent years.  Some paleoanthropologists have proposed that neither africanus nor afarensis were ancestral to the robust australopithecines.  In other words, the division between robust and gracile forms occurred earlier, perhaps at the time of anamensis or before.  It is also possible that humans descended from anamensis through a still unknown intermediate gracile species instead of afarensis.  On-going research will very likely sort out the relationships between the various hominid species in the near future.

http://anthro.palomar.edu/hominid/australo_2.htm

[Brooke]

Timeline of fossil hominids and their discoveries!

http://www.snowcrest.net/goehring/a2/primates/fossils.htm

[Brooke]

Bone Clones Catalog of Fossil Hominids. Skulls, tools, you name it!

http://www.boneclones.com/catalog_fossil_hominids.htm

[Jason]

Cool graphics, Brooke.