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Typical
human |
In addition to the great apes, the
family Hominidae
includes our species,
Homo sapiens
.
In the past, there also were other species of humans as well as hominids
more similar to us than the chimpanzees and bonobos. They will be
described in the last three tutorials of this series.
It has been historically difficult for people to accept that we are in fact just another primate species with African origins and that we differ physically only in degree from some of the others. The similarities can be seen throughout our bodies. For instance, humans and the African apes all lack external tails and have hands with a thumb that is sufficiently separate from the other fingers to allow them to be opposable for precision grips. Humans are also sexually dimorphic--males are 5-10% larger on average and have greater upper body muscular development. Like chimpanzees and bonobos, we are omnivorous. We kill other animals for food in addition to eating a wide variety of plants. Internally, our bodies are even more similar to the great apes. We have essentially the same arrangement of internal organs and bones. We share several important blood types. We also get many of the same diseases.
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| Similar chimpanzee and human hands | ||
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| Bipedal locomotion |
The comparatively minor anatomical differences between
humans and apes are largely a result of our habitual bipedalism
.
A number of changes in our bodies were related to the
evolution of this form of locomotion. Unlike apes, our arms are
relatively short and weak compared to our legs. Our feet no longer have the ability
to effectively grasp and manipulate objects because the
toes became shorter and the big toe moved up into line with the
others. Human feet also have lengthened and acquired an arch, making them better
body supports. The human pelvis and spinal column also have been modified for an erect posture and
efficient bipedal locomotion.
The pelvis became shorter, broader, and more bowl shaped. This
provided greater stability for walking and running.
We are now essentially fully terrestrial animals. Nature
very likely selected for longer legs in humans because it is more
efficient for walking and especially running bipedally. Research done
by Herman Pontzer of Washington University in St. Louis, Missouri indicates
that longer legs require less up-and-down movement while running and,
therefore, reduce the amount of energy needed to move rapidly. This
relatively lower rate of energy consumption would also allow humans to travel
farther with the same calorie expenditure. In addition, the largely
hairless human body with its abundance of sweat glands allows us to remain
cooler while running than if we only relied on panting like most other
mammals. This no doubt was a major advantage for our early human
ancestors in the competition with other hunters and scavengers for meat in
warm climates.
With the exception of these few outward differences, we are quite similar to the African apes anatomically and genetically, especially to the chimpanzees and bonobos. Humans have 46 chromosomes in their cells while all of the great apes have 48. In reality, this difference is not as great as it would initially seem because the human chromosome 2 is a fusion of ape chromosomes 12 and 13 with most of the same genes.
Human chromosome 2--video clip from Teachers' Domain
This link takes you to a new webpage. To return here, you must click the
"back" button on your browser program. (length = 3 mins 27 secs)
Work on discovering the entire genome of common chimpanzees was completed in 2005. A comparison between this and the human genome (completed in 2001) shows that 98.77% of DNA base pairs of humans and chimpanzees are the same. However, there are an additional 2.7% differences between the two species in duplicated non-protein coding segments of DNA. Where we differ appears to be largely in the genes that control speech, smelling, hearing, digesting proteins, and susceptibility to certain diseases. These minor differences are to be expected given that we have been on essentially separate evolutionary tracks for 6-7 million years. During that time, we have been subject to somewhat different natural selection pressures. These differences led to bipedalism for our ancestors along with a much larger brain and, ultimately, speech.
The modern human brain is 3 times larger in volume than those of the great apes. More importantly, the human brain to body size ratio is significantly larger, and it has a much bigger cerebral cortex with a higher concentration of neurons. Evolving a larger brain comes at a steep energy cost. The human brain uses about 25% of the energy derived from the nutrients that we consume and 20% of the oxygen. Recent research has suggested that our intelligence advantage may be due to evolutionary changes in the HAR1F regulator gene beginning about 6 million years ago in our pre-human ancestors but not in those of chimpanzees or other apes. This gene is involved in the production of brain tissue between the 7th and 19th week after conception. It is not surprising that there are some striking differences between the great apes and humans in mental abilities. People have much more complex forms of verbal communication than any other primate species. We are the only animal to create and use symbols as a means of communication. We also have more varied and complex social organizations. The most distinctive feature of humans is our mental ability to create new ideas and complex technologies. This has proven invaluable in the competition for survival. However, the great apes are remarkably intelligent, having mental levels equivalent to a 3-4 year old human child. This is sufficient to allow them to learn and use the sign language of deaf humans in at least a rudimentary way, but they do not have the capability of producing human speech and language. This is likely due to the fact that they have a different form of another key regulator gene known as FOXP2.
There are two additional curious differences between humans and all other primates that are worth noting. We are the only primate species that now lacks a thick insulating fur over our entire bodies. This has allowed humans to cool efficiently by the evaporation of sweat when it is hot and we are exerting considerable energy in running and other strenuous physical activities. We are also the only species of primate in which all older females go through menopause and become sterile, often decades before dying of old age. Female chimpanzees, gorillas, and other non-human primates usually remain capable of conception and giving birth even when they are very old. In the wild, they usually live a relatively short amount of time following menopause if they go through it at all. One explanation for this difference in humans is that years of life following menopause has proven to have natural selection value for our species. Having raised their own children, post-menopausal women around the world often take care of their grandchildren while their daughters are working. It is argued that this increases the chances that the grandchildren will survive to adulthood because they receive this additional experienced and caring attention.
This link takes you to a new webpage. To return here, you must click
the "back" button on your browser program. (closed caption version)
(length = 7 mins 27 secs)
Let us review the classification of apes and humans. Both are members of the suborder Anthropoidea, the Infraorder Catarrhini, and the superfamily Hominoidea.
ANTHROPOIDS
| suborder: | Anthropoidea | ||
|---|---|---|---|
| infraorder: | Platyrrhini | Catarrhini | |
| superfamily: |
Ceboidea |
Cercopithecoidea | Hominoidea |
| species: | New World monkeys | Old World monkeys |
apes humans |
Within the superfamily Hominoidea, there are two families. People are closest to the African apes genetically, especially the chimpanzees and bonobos.
HOMINOIDS
| superfamily: | Hominoidea | ||||
|---|---|---|---|---|---|
| family: |
Hylobatidae |
Hominidae | |||
| subfamily: | Ponginae | Gorillinae | Homininae | ||
| tribe: | Panini | Hominini | |||
| species: | gibbons |
orangutans |
gorillas |
chimpanzees bonobos |
humans |
News: The genome of rhesus macaque monkeys has been completed by a consortium of 23 laboratories (Science April 13, 2007). Comparing this genome with those already established for chimpanzees and modern humans will provide an even better tool for understanding the similarities and differences between the major groups of primates. Preliminary analysis indicates that macaques are similar to humans in 93% of their DNA sequences, while chimpanzees share about 98% of their DNA with us. This is to be expected since the last common ancestor of macaques and humans was about 19 million years ago, while the chimpanzees and human evolutionary lines diverged only around 6-7 million years ago. The U.S. National Human Genome Research Institute (NHGRI) has plans to sequence the genomes of marmosets, northern white-cheeked gibbons, orangutans, and gorillas.
This link takes you to 3 short videos at an external website.
(closed caption version)