The evolution of the genus Homo

Fig.3 The Olduvai Gorge in Tanzania where fossils of Homo Habilis, Homo Erectus and Homo Sapiens have been found

Courtesy רנדום via Wikimedia Commons

Introduction

Human Evolutionary Tree

The Species of the genus Homo

Homo Habilis

Homo Erectus

Homo Neanderthalensis

References

Introduction

All biological organisms are classified into groups based on their characteristics. This process is called Taxonomy. An organism will belong to a specific kingdom (plant or animal), a phylum, such as Arthropoda (Ex: centipede or arachnid) or Chordate (Ex: mammals, fish, birds), class (Ex: mammals), order (Ex: primates), family (Ex: Hominidae), Genus (Ex: Homo) and Species (Ex: Sapiens). Humans belong to the Animal kingdom, the Chordate phylum, the Mammal class, the Primate order, the Hominidae family, the Homo genus and the Sapiens species.
The Gorilla differs from the human only in the genus, which is Gorilla and the species which is Gorilla. The chimpanzee differs in the genus, which is Pan and the species which can be Troglodytes or Paniscus. Some scientists have called for a top level group called Domain which can be Archaea, Bacteria or Eukarya. Eukarya have cell nucleus; the other two are single cell microorganisms.

The scientific classification of humans, chimpanzees and gorillas is shown below.

The close relationship between humans, gorillas and chimpanzees is also evident from their DNA. About 99% of the DNA is common between humans and chimpanzees. Gorillas and humans share about 95% of the DNA.

The close relationship between humans, chimpanzees and gorillas have been know for several decades. However, the discovery in 1974 of a skeleton in the Afar region of Ethiopia by Donald Johansson and his colleagues expanded our understanding of human evolution. The skeleton called "Lucy" or "Dink'inesh" was a biped female. Based on argon-argon dating it lived about 3.2 million years ago. (The method of dating skeletons that are millions of years old will be explained in a later section). It was about 1.1m (3'7") tall and weighed about 29kg. The creature had a small brain like a chimpanzee but the pelvis and leg bones were similar to modern humans leading to the conclusion that it was a biped. Some researchers believe that Lucy was also arboreal ("tree dwelling").

In the picture below, a reconstruction of the parts of the skeleton that were discovered in the Afar region in the 1974 expedition is shown on the left. The picture on the right shows a reconstruction of a skeleton based on the fossil.

The Institute for Human Origins has an excellent page titled Lucy's story. The page explains how her gender, biped nature, the age of the skeleton etc. were determined.

Fig.4 Reconstruction of Lucy
Museum national d'histoire naturelle, Paris

Courtesy - Wikimedia Commons CC BY-SA 3.0

Fig.5 Reconstruction of the skeleton based on the fossil
Cleveland Museum of Natural History

Courtesy -
Andrew from Cleveland, Ohio, CC BY-SA 2.0 via Wikimedia Commons

Lucy was classified as Australopithecus Afarensis (Australo - south, pithecus - ape, Afar - region in Ethiopia where the skeleton was found). Lucy was not the oldest skeleton found in Africa. Ardipithecus ramidus (Ardi - ground / floor, ramid- root ) found in Ethiopia is 4.4 million years old, Australopithecus anamensis discovered in Kenya is 4.2 million years old and Australopithecus bahrelghazali discovered in Chad is 3.6 million years old.

These points must be noted -

No fossil of the genus Australopithecus have been found outside Africa
No fossil of the species Homo Habilis have been found outside Africa though some scientists believe the fossils in Dmanisi in Georgia could be H.Habilis
Homo Erectus did migrate out of Africa. Fossils have been found in Georgia, Java and China
The Neanderthals, whose fossils have been found in Europe and Asia and H.Sapiens are supposed to have evolved from Homo Heidelbergensis
Contrary to misinformation from religious zealots, no paleoanthropologist has stated that humans evolved from apes. The scientific evidence shows that humans and apes had a common ancestor and the split between the two happened over a period of time between 7 to 13 million years ago

Over the last 50 years, the large number of ape like and human like skeletons have been arranged into a human tree. This is not final and will see changes as more skeletons are found, dated and compared with the existing skeletons. The Human Origins Initiative at the Smithsonian has one such human tree. Mouse over each of the branches in the link above for more details on the various groups.

Human Evolutionary Tree

Fig.6 The human evolutionary tree

The Species of the genus Homo

While the study of Australopithecus, Paranthropus etc. is continuing, this site will focus on the genus Homo since that will lead us to the answer to the question posed at the beginning - Where do present day humans come from?

The genus Homo has several species. These include us, Sapiens, and several extinct species including Homo Habilis ("Handy Man"), Homo Erectus ("Upright Man"), Homo Neanderthalensis, Homo Heidelbergensis etc. Homo Sapiens is the only species of the genus Homo that survives today.

Homo Habilis

The first Homo Habilis fossil was discovered in the early 1960s by a team of anthropologists including Louis and Mary Leakey in the Olduvai gorge in Tanzania. H.Habilis lived about 2.4 million years to 1.4 million years ago (Mya) in Eastern and Southern Africa. It was thought the fossil was that of Australopithecus Afarensis but there were sufficient differences to consider this skeleton as belonging t0 a different genus and species. It had a larger braincase, smaller face and teeth. However it retained some ape like features such as long arms and a prognathic face (jaw protruding forward beyond the plane of the face). H.Habilis were thought to be about 3'4" to 4'5" tall and weigh about 70 kg. Their brain capacity was about 640 cc. H.Habilis was probably the first, or among the first, species to use tools made of bones and stone - hence the name. However, it is not certain if there were other older species using tools for hunting and cutting. There is a school of thought that H.Habilis is the ancestor of Home Erectus and then us - Homo Sapiens. However there is a Homo Erectus fossil that is older than a H.Habilis fossil which means they could have co-existed for thousands of years, much like Homo Sapiens and Neanderthals co-existed 40,000 years ago.

Homo Erectus

The first Homo Erectus fossil was discovered in 1891 by Dutch scientist Eugene Dubois in Java, Indonesia. The most significant fossils were found in China at the Zhoukoudian site by Swedish geologist Johan Andersson. They lived about 1.8 Mya to 110,000 years ago. When compared to H.Habilis, they appear to have a flatter face and a more prominent nose, much like H.Sapiens. Their height ranged from 4'9" to 6'1" and they weighed about 70 kg. Their brain capacity was about 900cc. Hand axes and cleavers have been found along with H.Erectus fossils signifying a greater usage of tools than H.Habilis. This is called the Acheulean tool industry. It is believed that H.Erectus used fire to cook food and seek warmth. They were the first member of the genus to migrate out of Africa to Asia (China) and Europe (Dmanisi in Georgia). The Dmanisi hominins are the oldest fossils found outside Africa. Several skulls and skeletal bones have been discovered. The fossils point to a shorter stature and lighter body. The tools found at Dmanisi appear to be more primitive than the hand axes and cleavers found with H.Erectus fossils in Africa. This along with the smaller brain capacity of the Dmanisi fossils suggests that these fossils were closer to H.Habilis than H.Erectus. This article in Science describes the Dmanisi fossils in detail and touches upon the difference of opinion among paleoanthropologists who have studied them.

H.Erectus is supposed to have lived for almost 1.8 million years. By contrast, H.Sapiens are supposed to be about 300,00 years old. One reason for this long duration is the grouping of fossils found in Dmanisi which is about 1.7 million years old and the Peking Man which could be as recent as 200,000 years old. It is possible that the Dmanisi fossils are probably more H.Habilis than H.Erectus. This would shorten the lifetime of the H.Erectus species.

H.Erectus is synonymous with Homo Ergaster.

Homo Neanderthalensis

Neanderthals are the closest species to humans. Neanderthals became extinct about 40,000 years ago. The first fossils that were identified as a different species was discovered in Neander valley in Germany in 1829. Fossils have been found in Europe, Southwest and Central Asia. At 1600 cc, their brain capacity was larger than those of Homo Sapiens. They were about 5'5" tall and weighed about 65kg making them shorter and stockier than H.Sapiens. Research has found that they used fire for cooking, warmth and protection and also buried the dead - the first species to do so. They wore clothing and due to the lower availability of plant food, compared to Africa, Neanderthals hunted large animals. Wooden spears have been discovered along with the fossils. There is evidence of detachable tools - different flakes could be fixed to a common stone core.

The Neanderthal Genome Project has found that 99.7% of the genome is identical between humans and Neanderthal. In addition, humans carry about 1%-4% of Neanderthal genes. It is not clear if there was breeding between Neanderthals and H.Sapiens. Humans and Neanderthals are supposed to have a common ancestor - Homo Heidelbergensis. It is believed that the split between H.Sapiens and Neanderthals could have happened about 400, 000 to 700,00 years ago.

Using statistical algorithms and skulls of various species including Homo Erectus, Neanderthal and modern humans, scientists have created a virtual (digital rendering on a computer) skull of the species that could be the ancestor of H.Sapiens and Neanderthals. This digital skull has an elongated head and heavy thickset brow, both of which are missing in H.Sapiens. It looks more like the Neanderthal skulls that have been discovered rather than that of H.Sapiens. Researchers believe this is understandable since H.Sapiens skull has deviated from those of its ancestor.

Neanderthals migrated to Europe and other parts of Asia about 400,00 years ago while H.Sapiens remained in Africa and migrated out of Africa later - about 70,000 to 100,000 years ago

Several reasons have been suggested for the extinction of Neanderthals. H.Sapiens moved into their areas around 40,000 years ago. It is postulated that they were better hunters and had a societal structure that helped them overcome Neanderthals who were smaller in number. Disease brought by the newcomers from Africa might have been another reason.

Neanderthals are believed to be the first species to have left behind art. Cave paintings in Spain and engravings on cave walls in France are attributed to Neanderthals because they are dated to 40,000 and 57,000 years ago. However, these could have been made by H.Sapiens as well, especially the paintings in Spain, since H.Sapiens are thought to have reached Europe around that time.

This table shows a comparison between H.Habilis, H.Erectus, H.Neanderthalensis and H.Sapiens

This section has looked at the various species that came before us along with the dates when they existed. The next section will look at how fossils are dated.

The evolution of the genus Homo

Introduction

Human Evolutionary Tree

The Species of the genus Homo

Homo Habilis

Homo Erectus

Homo Neanderthalensis

References

See Also