The Descent of Man | Study Guide

Charles Darwin

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The Descent of Man | Chapter 1 : The Evidence of the Descent of Man from Some Lower Form | Summary

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Summary

The Descent of Man, while sometimes published as a separate volume, was originally the first part of a longer work called The Descent of Man; and Selection in Relation to Sex. The full title of Part 1 is "The Descent or Origin of Man." The second and third parts of the longer work deal with the topic of sexual selection.

Darwin opens the chapter with the claim that in order to determine whether humans are a descendant of a previous life-form, some important questions must be addressed. Do humans share similarities in brains or bodies with lower animals? Are humans governed by the same general laws as other organisms? Have humans produced subspecies like other animals? Do such subspecies encroach on each other, resulting in extinction? Darwin asserts that the answer to such questions is yes—humans descended from lower forms.

He organizes the chapter around three primary areas of comparison: 1) the bodily structure of humans; 2) embryonic development in humans and animals; and 3) rudiments (body parts or organs that develop later or differently or nearly uselessly). Darwin spends most of the chapter examining these rudimentary comparisons between humans and animals.

Humankind shares the same bodily structure model of bones, muscles, blood vessels, and brains as most other mammals, though Darwin notes there can be stronger similarities between humans and certain other mammals, such as orangutans and baboons. He notes that humans and monkeys share similar diseases, such as consumption (tuberculosis) and scabies (skin rash), indicating related similar tissue and blood structures. Darwin cites studies by contemporary anatomists that show monkeys' nervous systems are affected similarly by coffee, tobacco, and liquor. Reproduction is also related in humans and monkeys: female monkeys' menstrual cycles follow the moon cycle, males court potential female mates, and newborn monkeys are nearly as helpless at birth as human babies. He points out that the physical and mental differences between male and female monkeys correspond to the differences between their human counterparts.

The embryonic development of a human also shares remarkable similarities to other mammals. The structure of the human ovum, or egg, is indistinguishable from that of other animals. He describes the similar features of dog and human embryos, noting that the eyes, ears, brains, and arms and legs are nearly identical. Later stages of embryonic development will reveal the differences between mammals.

Darwin dedicates most of the chapter to examining rudiments, which are organs and body parts that are either completely useless or serve such a minimal purpose that anatomists aren't sure why or to what purpose they were originally developed. All higher animals have such rudimentary organs, which Darwin suggests developed as a result of disuse. Reduced muscle activity and blood flow to an organ or body part over time renders it useless. These rudiments are key indicators, according to Darwin, of humankind's structural similarities to lower animals because so many are shared. They also reinforce genetic evidence of heredity between families (both human and animal). Darwin cites a family whose members—at least eight generations of them—were able to move their scalps by activating the muscles below the skin. It's a useless (rudimentary) function that serves no clear purpose, but at some point earlier humans (and monkeys) must have been able to do this perhaps for a reason.

One example Darwin uses is the ear. Many animals can move their ears directionally; humans and monkeys cannot, and the two share very similar structures, including the shell around the ear, which is rudimentary and doesn't move. Darwin suggests that because humans and monkeys can easily turn their heads from side to side, their ears are no longer needed to sense danger, as dogs' and horses' ears are still needed. The sense of smell is another notable difference: humans' sense of smell is used primarily for memory recall and tasting food, whereas dogs—who have a nearly identical embryonic structure to humans—use smell to identify people and places. Other examples of the human rudimentary condition that Darwin examines include wisdom teeth, the appendix, the tailbone, and male mammary glands. The fact that so many mammals (especially monkeys) share these same rudiments leads Darwin to conclude that "man and all other vertebrate animals have been constructed on the same general model" and that it is only "arrogance, which made our forefathers declare that they were descended from demigods." This arrogance prevents some people from admitting our evolutionary connection to lower mammals.

Analysis

Appearing 12 years after his On the Origin of Species (1859), The Descent of Man is Darwin's continued examination of his theory of evolution, benefiting from copious amounts of additional research and data published by other scientists inspired by On the Origin of Species. Darwin makes it clear in the first paragraph of the chapter that he will be addressing some elemental questions in determining humankind's descent. He presents the larger questions he wants to address and then proceeds to present his findings in three elemental areas: the bodily structure of man, embryonic development, and an in-depth examination of rudimentary organs and body parts.

Darwin provides much evidence from other naturalists of the time, both to support the ideas he is addressing and to make it clear to the reader that researchers other than himself have found additional evidence supporting his theories. He uses detailed footnotes throughout the chapter to provide context for the research he is citing.

Darwin's overarching assertion is that humans are a descendant of a preexisting form, so he begins his analysis by showing how mammals of all types share so many similar structures (blood, tissue, bones, lungs, etc) at all stages of their lives. This suggests that these mammals must share common ancestry—how else could they be so similar at conception?

The most in-depth area of examination is rudiments—animal body parts or organs that develop later or differently or are nearly useless. Darwin uses these rudiments to show how similarly structured embryos develop based on natural selection. He provides evidence of rudiments that are different in man and other animals. For example, horses and dogs can move their subcutaneous (below the skin) muscles to make their hide and fur twitch—humans and monkeys cannot. But his strongest argument is examining rudiments that are similar in man and animals. For example, in the womb, human and monkey fetuses have fine hair covering their skin, though humans are far less hairy when born. But both humans and monkeys have completely hairless palms and soles of their feet—like most other mammals. Again, Darwin is bolstering his case for the direct structural link between humans and other animals.

On two occasions in Chapter 1 Darwin makes mention of differences between darker- and lighter-skinned humans. In the mid- to late-1800s, post-slavery in Europe and the United States, much was written and researched about the differences between light- and dark-skinned people (sometimes referred to as primitive or savage). Darwin makes an observation that could be interpreted as falling into the more racially biased theories of his day: dark-skinned people's wisdom teeth are more sound and necessary than those of light-skinned ("civilized") people because of the latter becoming accustomed to eating soft, cooked food.

Darwin closes the chapter with what can only be interpreted as something of a jab at detractors who had reacted so negatively to his theory of evolution 12 years earlier. He sarcastically suggests that believing each human is the result of a creator's act seems ridiculous in light of the science.

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