Human Evolution




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Human Evolution


1. Describe primate origins and characteristics

2. Discuss the development of Hominids

3. Compare the two models for the origin of modern Homo Sapiens

Illustrations are included in the

online version of the notes at:


< http://www2.cajun.net/~ths/science/b2humev.htm >

Modern humans are in the primate family Hominidae. We are the only living members of that family today. In the past, australopithecines and other hominid genera (as well as other Homo species) have lived but are now extinct.



Note: The evolutionary theory of human developement is perhaps one of the most volatile of the science world. Every few years, new ideas and fossil evidence surfaces and causes changes in theories. You can ask 5 leading paleoanthropologists of today to diagram the human family tree, and you'd probably get 5 different versions.

Genus Australopithecus


Australopithecines are thought to be the link between Ardipithecus ramidus and the Homo genus. Some scientists put several larger species of australopithecenes into their own genus, Paranthropus. Up till now, ardipithecus and australopithecene fossils have only been found in Africa. Although most have been found in the southern and eastern parts of the continent, some have recently been found in the north-central country of Chad. The first species in Homo, the Homo habilis, has also been found only in Africa. Some believe that the next step in the hominid tree splits in two. Those that stayed in Africa were Homo ergaster, while Homo erectus is thought to be the first hominid to expand its range beyond Africa.

Besides the major Australopithecene species detailed below, there are other where minor fossil evidence has been found. The fossils found in Chad have been named Australopithecus bahrelghazali. This species lived from 3.5 to 3 million years ago. Since the fossil evidence is limited, we don't know much about Australopithecus bahrelghazali. Also, some fossils found in east Ethiopia (dated about 2.5 million years old) have been called Australopithecus garhi. Some say that a. garhi is a good candidate as an intermediary between australopithecenes and Homo habilis. Fossils found within the last 5 years seemingly push back the hominid family tree even further, with Sahelanthropus tchadensis and Orrorin tugenensis dating back 6 million years.

Generally speaking, the Australopithecenes were thought to be our primitive ancestors ... the first that started the switch to bipedalism and larger brains (among other characteristics). In the human family tree, they are supposed to have been followed by the Homo species. Homo habilis came first, followed by Homo erectus/ergater. Homo erectus/ergaster was followed by Homo sapiens. Homo neanderthalensis is often thought to be a "cousin" to modern man. Some think that Homo heidelbergensis may have been the common ancestor to both modern humans and Neanderthals. Check the table at the bottom of the page for a summary of most hominid species.

Hominid characteristics: there are certain features that place a species in the Hominidae family. The more advanced they appear, the closer the species is to modern humans.

- Bipedalism - walking upright; evidence for this includes a change in the femure/pelvis structure, the foramen magnum placed under the cranium, and shorter arms; early australopithecenes seem to show that they were starting to walk upright part of the time

- Brain size - modern humans average about 1350cc in brain size; increasing brain size of hominids may indicate evolution towards H. sapiens; a more vertical forehead indicates the development of the frontal lobe of the brain

- Speech - the ability to speak is a hominid characteristic; measuring brain casts tells us which hominids may have been able to speak; the first words may have begin with H. habilis

- Tools - hominids are thought to be the first animal to make and use tools; the first tools were thought to have been made by H.habilis


NOTE: both Sahelanthropus tchadensis and Orrorin tugenensis predate A. ramidus, but their discovery is so recent that there is not much written on how they may fit into the hominid family tree.


Ardipithecus ramidus


There is only one species in this genus, A. ramidus (sometimes A. ramidus kadabba). Fossil evidence dates back to about 4.5 mya. This, perhaps the earliest hominid, was identified from fossils found around Aramis, Ethiopia in the 1990s. This individual was probably part of a radiation that included other hominids. Because of its recent discovery, not much has been written about it. If you take a look at the fossils recovered thus far [picture], you can see how difficult it might be to speculate as to it's skull characteristics.

The specimen’s hominid characteristics suggest that it was 'on the road ' to being bipedal. The characteristics include a valgus knee angle, anteriorly placed foramen magnum and most importantly, a short, broad pelvis.



Australopithecus


Australopithecenes are considered to be the progenitors of the Homo genus. There are two main types: gracile and robust.

Gracile australopithecenes: Their skeletons are less "bony." For example, they do not have bony crests on their skulls to support large chewing muscles. Their overall size was less than the robusts. They started to appear about 4 million years ago and lasted till about 2 million years ago. Gracile species include A. afarensis, A. africanus, and A. anamensis.

Robust australopithecenes: An overall larger bone structure gives these species the robust title. Their large molars and jawbones may mean that their diet required tough chewing. They started to appear about 4 million years ago and lasted till about 1 million years ago. Robust species include A. aethiopicus, A. boisei and A. robustus. Some scientists think that they are different enough from australopithecenes that they deserve their own genus, Paranthropus.

Australopithecus anamensis


Australopithecus anamensis may have been the earliest australopithecine species. They lived 4.2-3.9 million years ago in East Africa. We know little about them because few fossils have been found. It may have been partially bipedal.

Australopithecus afarensis


Australopithecus afarensis lived 3.9-3.0 million years ago in East Africa (Ethiopia, Tanzania, Kenya.). Their skeletons were still quite similar to early Pliocene apes. For example, their legs were shorter than the later australopithecines. They had large canines. Some of the males had small sagittal crests. Afarensis had slender curved fingers like modern chimpanzees. Because of these features, they may have been more adapted to tree climbing than bipedalism. They were under 5 feet tall and weight about 100 pounds.

The females were significantly smaller (weighed 64% as much as males and were 70% of the males' height). Their slender structure also has led to the australopithecenes being known as gracile species.




Genus Homo


The first member of the Homo genus is commonly thought to be Homo habilis (and/or Homo rudolfensis). There is some disagreement as to how things evolved afterwards. Homo ergaster and Homo erectus are thought to follow H. habilis. H. ergaster stayed in Africa, while H. erectus moved to the Asia about 2 million years ago. While we know that modern humans are classified as Homo sapiens, the exact lineage is uncertain.

For a long time, it was assumed that we came from H. erectus. Neanderthals (H. neanderthalensis) is thought to be a 'cousin' of H. sapiens. Recent thought is that Homo heidelbergensis may be a common ancestor to H. sapiens and Neanderthals.

It is thought that H. habilis may have been the first tool makers. H. erectus advanced the art of toolmaking and was the first to utilize fire.


Homo habilis / Homo rudolfensis


Though many paleoanthropologists classify Homo habilis ("handy man") as a Homo sapiens ancestor, the exact bridge from one species to another has been debated since the Leakeys found the first specimen (OH 7) in 1960. Homo habilis is thought to be the first hominid to use simple tools. Its brain size is bigger than that of the australopithecenes. Experts can't even agree on exactly which specimens should be definitely considered Homo habilis.

Adding to the confusion over Homo habilis, some specimens have sufficient differences that another species has been proposed. V.P. Alexeev, using thee KNM-ER 1470 as a type specimen, suggested the species Homo rudolfensis in 1986. Homo rudolfensis may have been the ancestor to the Homo habilis. Perhaps they were two separate species. Some even believe it should be classified as an australopithecene.




Homo erectus / Homo ergaster


The Leakeys have long proposed that Homo erectus was the ancestor of modern Homo sapiens. Other opinions do exist (see Homo sapiens below). One theory is the Homo erectus left Africa to populate Asia and perhaps Europe, but Homo ergastor stayed in Africa and evolved into Homo sapiens who then replaced Homo erectus. Of course, if you believe in the multiregional theory of H. sapiens origins, then you might say that H. ergaster evolved into a form of H. erectus and then into H. sapiens in Africa, while H. erectus evolved into H. sapiens in other parts of the world.

Homo erectus is thought to have advanced the art of tool making. They are also thought to be the first hominids to use fire. Their brain capacity averaged over 1000cc (while H. habilis averaged 750cc).

Homo heidelbergensis


Homo heidelbergensis fossils have been found in Africa and eastern & western Europe. Their fossils date from about 600,000 to 200,000 years ago. It's features are similar to Homo erectus, but its other features, the time frame, and location seem to indicate that it may have been the ancestor to Neanderthals and possibly H. sapiens.

Homo sapiens


This is "us." The modern traits of today's human started appearing about 100,000 years ago (though recent discoveries may push that back another 50,000 years). The earliest fossil record of Homo sapiens is from Africa. Sometimes early Homo sapiens is known as archaic or Cro-Magnon man.

Brain size is about 1350cc and we are fully bipedal. Although we have hair over most of our bodies, at some point we lost the thick coat we once had. [see Why Humans and Their Fur Parted Ways] Theories range from the idea that humans went through a semi-aquatic phase and lost it ... to the idea that it was lost through natural selection (more hair = more parasites & diseases).


Two models for the origin of the species, Homo sapiens

Just as there is disagreement over the order of the entire hominid family tree, there are 2 different schools of thought on how our species came to be. These 2 theories that developed in the 1980s are known as the 'Out of Africa' theory and the multiregional theory.


'Out of Africa' Theory:

- This idea says that Homo sapiens evolved in Africa, and THEN spread out to the whole world.

- These Homo sapiens would replace any existing populations that were their (e.g. Neanderthals in Europe).

- This is the leading theory because of the hominid fossils and fossil artifacts found in Africa. Also, mitochondral DNA studies (1987 et. al) suggest a common ancestor in Africa (the "Eve" hypothesis). But recent DNA research is casting doubt on this idea. [see Is Out of Africa Going Out the Door?]

- The different races are a more modern development.
Multiregional Continuity theory:

- This idea says that about 7 mya, Homo erectus left Africa and settled in different parts of the world.

- They (or their descendants) evolved into Homo sapiens in those different parts of the world.

- For example, Homo erectus evolved into Homo sapiens in Asia, Homo ergaster evolved into Homo sapiens in Africa, Homo neanderthalensis evolved into Homo sapiens in Europe, etc.

- Natural selection in those different areas led to the variations in Homo sapiens races.

- One of the problems with this theory is that in some areas, Homo sapiens existed at the same time as their proposed ancestors.


These hominids currently thought NOT to be in our direct human lineage.

Australopithecus africanus


Until 1925, it was thought that human ancestors would only be found in Europe. But that year, Raymond Dart labeled his find (the Taung child) as a hominid because it was partially bipedal and created the genus australopithecus ("southern ape").

Australopithecus africanus lived about 3.0-2.5 million years ago in East and South Africa. Their bone structure was less ape-like than earlier species of australopithecines. Their skeletons were still small and lightweight ... even smaller than A.afarensis. Unlike A. afarensis, their canines did not project beyond the other teeth.


A. robustus


Some scientists give the larger australopithecenes their own category (paranthropus), though some paleoanthropologists still call them australopithecenes.

Paranthropus or A. aethiopicus - the earliest robust species; lived about 2.5 million years ago; found only in East Africa; smaller brain than the other robust species; large sagittal crest.

Paranthropus or A. boisei - the largest of the three Paranthropes; found in East Africa; lived about 2.3-1.4 million years ago; large canines and grinding teeth

Paranthropus or A. robustus - lived about 1.9-1.0 million years ago in South Africa; smaller version of the p. boisei
Homo sapeins neanderthalensis or Homo neanderthalensis

Neanderthals started appearing about 130,000 years ago, and they seem to have died out about 30,000 years ago. Their fossil evidence is mainly in western Europe. Some people think that rather than

becoming extinct, surviving Neanderthals mixed in with the Homo sapien population.


Comparative Brain Sizes:

A. afarensis H. habilis H. erectus H. sapiens neand. H. sapiens Chimp Gorilla

400cc 750cc 900-1200cc* 1400cc 1350cc 400cc 500cc


*900cc for early specimens (>1 mya), 1200cc for later ones (1/2 mya)

The Hominid Family Tree


Here is one of the more commonly accepted views of the hominid family tree. Other versions can be found at the Hominid Family Tree page online < http://www2.cajun.net/~ths/science/b2humevtree.htm >.


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