Sarim Baig

Evolution in Humans and the Thoughts of Major Evolutionary

Scientists

Evidence for Human Evolution

• Homology

• Vestigial Organs• Appendix, Tailbone, and others

• Paleoanthropology• Similarities between humans and others in the “Homo”

genus, as well as earlier species.

Homology

• Mammalian Forelimbs• Most often cited example of homology• Humans differ greatly from other mammals such as bats and

whales yet have almost the exact same bone structures in the forelimbs.

• Single bone attached to two bones which are then attached to multiple bones.

• Each of these structures was slightly modified in order for the organism to adapt to its surroundings.

• Same bone structure allows species to fly, swim, run, etc.

Homologous Structures

Vestigiality

• Structures whose physiological roles in the organism have either been diminished or have disappeared altogether.

• Human tailbone:• Other organisms use the tailbone as a means for balance,

communication, and even movement.• Other organisms such as dogs have a much larger and

more pronounced tailbone than humans.• Human tailbone is a small “hook” at the end of the vertebral

column and has no function in movement and/or balance.

Human Appendix

• The human appendix is a “worm-like” structure found in the digestive system at the end of the “caecum”.

• Gradually shrinks in humans as we age.• The appendix can be removed surgically without any

major side effects.• Other mammalian organisms such as rabbits also

have an appendix, yet their appendix has a defined function.

The function of the appendix

• Appendix and caecum play a vital role in the digestion of cellulose in herbivorous animals.

• In humans, enzymes that are used to digest cellulose are found in low quantities in the caecum and are mostly absent in the appendix.

• In rabbits, the appendix plays a major role in its immune system• Half of its lymphoid tissue found in appendix• Majority of human lymphoid tissue is found in the small and

large intestines.

Figure: Rabbit Appendix (Left), Fetal Human (Middle), Adult Human (Right)

Vestigial Organs: Appendix

Other Vestigial Structures in Humans

• Wisdom Teeth?• Many individuals never have their wisdom teeth come in.• Many are often impacted and need to be removed.• Generally not used for biting and/or chewing.

• Genes for Vitamin C- Ascorbate• The human genome has the majority of the genes needed to

produce vitamin C internally, yet humans are incapable of doing this.

• Evolved this trait because early humans consumed a lot of vitamin C in their everyday diet.

Paleoanthropology

• Study of early hominids and how their biological and cultural characteristics are related to modern-day humans.

• Majority of the evidence in this field came from studying the fossil record.

• Earliest definitive hominids were of the Australopithecus group, which lived around 4 million years ago.

• Two major trends are seen in the history of hominids.• Evolution of bipedalism• Greater intellectual capacity

Bipedalism

• The ability to walk on two legs is directly related to the length of the arms relative to the legs.

• Species with longer arms and shorter legs are much more likely to be quadropeds.

• Species with shorter arms and longer legs are more likely to be bipedals.

• Humerus to Femur ratio

• Chimps: ~ 1 : 1• A. afarensis (Lucy): ~ 0.85 : 1• Humans: ~ 0.72 : 1

Intellectual Ability

• The overall intellectual ability of mammalian species has been attributed by some to be directly related to the size of the organism’s brain.

• Brain sizes in hominids have gradually increased over time.

• Led to the development of tools in later hominids and other innovations (Fire, agriculture).

Intellectual Ability Continued

• A. afarensis: ~ 400 cc• Lived 4 m.y.a• Approximately the same size as chimps and other apes.

• A. boisei: ~ 530 cc• Lived 2 m.y.a

• Homo erectus: ~ 900 cc• Lived up until 300,000 years ago

• Homo neanderthalensis: ~ 1500 cc• Lived up until 30,000 years ago

• Homo sapiens: ~ 1350 cc

Chimpanzee Australopithecus afarensis

Australopithecus boisei

Homo erectus Homo neanderthalensis Homo sapiens

Trends in Hominid Brain Size

Differences between Early Hominids and Late Hominids

Charles Darwin

• Witnessed the controversy that followed the publication of his paper and Wallace’s in 1858.

• Reluctant to mention anything about evolution in humans in the Origin of Species (1859).

• Waited twelve years before making any publication dealing with the issue of human evolution.

• Published The Descent of Man after a number of other scientists had already brought the topic into the public eye.

The Descent of Man (1871)

• Cites homologous structures, vestigial organs, and comparative embryology as evidence that humans evolved from earlier species.

• “…that man is descended from some lowly organized form, will, I regret to think, be highly distasteful to many. But there can hardly be a doubt that we are descended from barbarians.”

Alfred Russel Wallace

• Human evolution was a result of two forces, natural selection and the work of a spiritual being.

• Natural selection could not alone account for the “vast superiority of man over its nearest allies.”

• “The brain in savage and prehistoric man was in advance of his needs”

Thomas Henry Huxley (1825-1895)

• “Darwin’s Bulldog”

• First to publish on evolution in humans after reading Darwin’s Origin of Species

• Evidence on Man’s Place in Nature (1863)

• Much of Huxley’s work dealt with the similarities between humans and apes.

Huxley-Wilberforce Debate (1860)

• Oxford University• Archbishop Wilberforce:

known as a skilled debator

• Huxley: a relative unknown in the scientific community.

• Expected to be a defeat of Huxley and of Darwin’s ideas.

Results of Debate and Its Impact

• By most accounts, Huxley soundly defeated Wilberforce in the debate.

• Wilberforce is said to have asked Huxley which side of his family descended from apes.

• Huxley response was that he would rather be descended from apes than from a man afraid to face the truth.

• Huxley’s victory made believers out of many individuals in the public who had previously attacked evolution.

http://herc.berkeley.edu/middle_awash/what_is_paleoanth.php. Accessed Nov. 18, 2008.

http://www.evoled.org/lessons/human.htm#overview. Accessed Nov. 18, 2008.

http://cogweb.ucla.edu/ep/Paleoanthropology.html. Accessed Oct. 27, 2008.

http://www.nasw.org/users/karin/globe1.html. Accessed Nov. 22, 2008.http://www.talkorigins.org/faqs/vestiges/appendix.html. Accessed Oct. 27,

2008.http://www.fordham.edu/halsall/mod/1871darwin.html. Accessed Oct. 31,

2008.

References

References Continued

Wallace, Alfred Russel. Man and Natural Selection (S173: 1870). The Alfred Russel Wallace Page. Accessed Oct. 31, 2008.

Wallace, Alfred Russel. Darwin’s ‘The Descent of Man and Selection in Relation to Sex’ (S186:1871) The Alfred Russel Wallace Page. Accessed Oct. 31, 2008.

http://www.ucmp.berkeley.edu/history/thuxley.html. Accessed Oct. 31, 2008.

http://cas.bellarmine.edu/tietjen/Ec&Ev_Distance_learning/Evidence/evidence_main.htm

. Accessed Nov. 26, 2008.http://www.holyconservancy.org/images/history/1860ad/thomas-h-huxley.gif

. Accessed Nov. 26, 2008.