This figure illustrates 3 hypotheses of how a cancer stem cell may arise: (1) A stem cell undergoes a mutation, (2) A progenitor cell undergoes two or more mutations, or (3) A fully differentiated cell undergoes several mutations that drive it back to a stem-like state.
Human evolution
Human evolution is the lengthy process of change by which people originated from apelike ancestors. Scientific evidence shows that the physical and behavioral traits shared by all people originated from apelike ancestors and evolved over a period of approximately six million years.
One of the earliest defining human traits, bipedalism -- the ability to walk on two legs -- evolved over 4 million years ago. Other important human characteristics -- such as a large and complex brain, the ability to make and use tools, and the capacity for language -- developed more recently. Many advanced traits -- including complex symbolic expression, art, and elaborate cultural diversity -- emerged mainly during the past 100,000 years.
Humans are primates. Physical and genetic similarities show that the modern human species, Homo sapiens, has a very close relationship to another group of primate species, the apes. Humans and the great apes (large apes) of Africa -- chimpanzees (including bonobos, or so-called “pygmy chimpanzees”) and gorillas -- share a common ancestor that lived between 8 and 6 million years ago. Humans first evolved in Africa, and much of human evolution occurred on that continent. The fossils of early humans who lived between 6 and 2 million years ago come entirely from Africa.
Most scientists currently recognize some 15 to 20 different species of early humans. Scientists do not all agree, however, about how these species are related or which ones simply died out. Many early human species -- certainly the majority of them – left no living descendants. Scientists also debate over how to identify and classify particular species of early humans, and about what factors influenced the evolution and extinction of each species.
Early humans first migrated out of Africa into Asia probably between 2 million and 1.8 million years ago. They entered Europe somewhat later, between 1.5 million and 1 million years. Species of modern humans populated many parts of the world much later. For instance, people first came to Australia probably within the past 60,000 years and to the Americas within the past 30,000 years or so. The beginnings of agriculture and the rise of the first civilizations occurred within the past 12,000 years.
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Cellulose is another long polymer of glucose. Plant cells make their cell walls out of cellulose. In fact, 100 billion tons of cellulose is made every year on earth. Cellulose is indigestible in most animals, including us. Ever eat a cardboard box? You get the picture. We simply lack cellulase, the enzyme that can break it down. Some bacteria, some single-celled protists, and fungi have the enzyme. Animals that feed on cellulose harbor these microbes that help them digest it. Even though, we cannot break down this molecule, we do need cellulose in our diet. We call it “fiber”. Cellulose stimulates the colon to produce regular bowel movements and helps make the stools large and soft. A diet rich in fiber can prevent a painful intestinal disorder called diverticulosis. Hard impacted stools can sometimes cause the walls of the colon to form blind outpockets called diverticula which can periodically inflame. So what makes cellulose different from starch? Isn’t it made of glucose? Well it is but the glucose monomers are organized in an interesting fashion. The orientation of the glucose molecules alternates. So if the first one is right side up, the next one is upside down and then the next is right side up and the next one is upside down. Apparently this is a tricky arrangement for an enzyme to break.
The answer is b it is being bred for a trait that is favored by humans