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.
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 with powerful muscles and spring-like tendons 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. Humans can easily be outrun by many other animals over short distances. However, we are endurance runners and can ultimately run down virtually all other land animals.
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.