The Primates: Humans

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 today. 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.

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 development 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 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 bipedal locomotion. 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.

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.

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. 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.

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. 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.

Let us review the classification of apes and humans. Both are members of the suborder Anthropoidea, the Infraorder Catarrhini, and the superfamily Hominoidea.

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.

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.



	Typical human sexual dimorphism



	Bipedal locomotion