Common ancestors are the branching points in a phylogenetic tree, from which multiple species originate. Chimpanzees and humans share a common ancestor. Through evolution, the genetic changes to this ancestor gave rise to two separate species: chimpanzees and humans. This divide occurred roughly 5 million years ago, according to fossil evidence.

To say that chimpanzees are the most recent common ancestor to humans implies that humans actually evolved from chimpanzees. Similarly, to say that humans are the most recent ancestor to chimpanzees suggests that chimpanzees evolved from humans. Neither of these statements are correct, as both species are actually derived from a shared ancient relative (the most recent common ancestor).

There are three eras that divide geologic time: the Paleozoic era, the Mesozoic era, and the Cenozoic era. The Mesozoic era was the age of the dinosaurs. Following their extinction, the Cenozoic era began and mammals started to dominate the landscape. This gives the Cenozoic era the nickname "the age of the mammals."

A population will not experience evolution if all of the following requirements are met:

1. Very large population numbers.

2. No new mutations entering the population.

3. No new organisms entering the population and changing the gene pool.

4. All mating is random.

5. No natural selection.

These tenets are the foundation of Hardy-Weinberg equilibrium. If mating is no longer random, evolution will take place in the population. Similarly, if mutations occur, new organisms enter the population, the population size is small, and natural selection is in effect, then evolution will be enhanced.

The chief cells are responsible for releasing pepsinogen. Pepsinogen is an inactive enzyme that will only become activated in acidic conditions, such as those found in the stomach. Acid converts pepsinogen into pepsin, which helps chemically digest protein nutrients in the stomach.

Parietal cells release hydrochloric acid into the stomach, which activates pepsinogen. Goblet cells, or mucous cells, secrete mucous to help prevent degradation of the stomach lining.

The esophagus assists moving food into the stomach using a wave-like contracting motion called peristalsis. Swallowing initiates the process of peristalsis, but unlike swallowing, peristaltic contractions are not voluntary and are generated by smooth muscle.

Mastication is another term for "chewing." Emulsification refers to the grouping together of fat molecules when in an aqueous environment, particularly during digestion in the small intestine.

Food is digested to some degree in many different parts of the digestive system. The mouth, stomach, and small intestine all contribute to food digestion. The mouth contains salivary amylase, which breaks down carbohydrates. The stomach contains pepsin, which breaks down proteins. The small intestine contains several enzymes, such as lipase and trypsin, and breaks down carbohydrates, lipids, and proteins.

The esophagus, however, does not contribute to chemical digestion in any way.

Bile is created by the liver and released into the small intestine from the gall bladder during digestion. Because fat tends to clump in the aqueous environment of the duodenum, bile helps to increase the surface area of fat, a process called emulsification. This helps lipase break down the fats adequately.

Muscles do not attach directly to bones. Instead, a tendon is used to attach the two structures. Ligaments connect a bone to another bone, while joints are the locations where bones move around one another.

There are three types of muscle in the body: skeletal, cardiac, and smooth muscle. Of the three, skeletal muscle is the only one that can be consciously controlled. Cardiac and smooth muscle are controlled involuntarily.

An example of skeletal muscle is your biceps brachii muscle; you can voluntarily control this muscle to flex your arm. An example of smooth muscle is the muscle surrounding your arteries; you cannot voluntarily contract this muscle to constrict your arteries. Cardiac muscle is found in the heart; you cannot voluntarily choose to slow your heart rate.

Red bone marrow is found in the ends (epiphyses) of long bones such as the femur and is responsible for red blood cell development. Yellow bone marrow is found in the center (diaphysis) of long bones and can store fat for energy.

Blood filtration primarily occurs in the spleen or kidney. Glycogen is primarily stored in the liver. Calcium is released from the mineral matrix of the bone, but is not actually found in the bone marrow.