The word theory in the theory of evolution does not imply mainstream scientific doubt regarding its validity; the concepts of theory and hypothesis have specific meanings in a scientific context. While theory in colloquial usage may denote a hunch or conjecture, a scientific theory is a set of principles that explains observable phenomena in natural terms. “Scientific fact and theory are not categorically separable,” and evolution is a theory in the same sense as germ theory or the theory of gravitation.
Evolution does not attempt to explain the origin of life or the origin and development of the universe. While biological evolution describes the process by which species and other levels of biological organisation originate, and ultimately leads all life forms back to a universal common ancestor, it is not primarily concerned with the origin of life itself, and does not pertain at all to the origin and evolution of the universe and its components. The theory of evolution deals primarily with changes in successive generations over time after life has already originated. The scientific model concerned with the origin of the first organisms from organic or inorganic molecules is known as abiogenesis, and the prevailing theory for explaining the early development of our universe is the Big Bang model.
Humans did not evolve from chimpanzees. The two modern chimpanzee species are, however, humans’ closest living relatives. The most recent common ancestor of humans and chimpanzees lived between 5 and 8 million years ago. Finds of the 4.4 million year old Ardipithecus indicate the ancestor looked like a small, long limbed chimpanzee with a rather short snout and was a moderately competent bipedal walker. Contrary to the idea of chimpanzees as “primitive”, they too have evolved since the split, becoming larger, more aggressive and more capable climbers. Together with the other apes, humans and chimpanzees constitute the family Hominidae. This group evolved from a common ancestor with the Old World monkeys some 40 million years ago.
Evolution is not a progression from inferior to superior organisms, and it also does not necessarily result in an increase in complexity. A population can evolve to become simpler, having a smaller genome, but biological devolution is a misnomer.
According to the California Academy of Sciences, only 59% of U.S. adults know humans and (non-avian) dinosaurs did not coexist. The last of the non-avian dinosaurs died 65.5 million years ago, after the Cretaceous–Paleogene extinction event, whereas the earliest Homo genus (humans) evolved between 2.3 and 2.4 million years ago. This places a 63 million year expanse of time between the last non-bird dinosaurs and the earliest humans.
Evolution does not violate the Second Law of Thermodynamics. A common argument against evolution is that entropy, according to the Second Law of Thermodynamics, increases over time, and thus evolution could not produce increased complexity. However, the law does not refer to complexity and only applies to closed systems, which the Earth is not, as it absorbs and radiates the Sun’s energy.
Evolution does not “plan” to improve an organism’s fitness to survive.For example, an incorrect way to describe giraffe evolution is to say that giraffe necks grew longer over time because they needed to reach tall trees. Evolution doesn’t see a need and respond, it is instead a goalless process. A mutation resulting in longer necks would be more likely to benefit an animal in an area with tall trees than an area with short trees, and thus enhance the chance of the animal surviving to pass on its longer-necked genes. Tall trees could not cause the mutation nor would they cause a higher percentage of animals to be born with longer necks. In the giraffe example, the evolution of a long neck may equally well have been driven by sexual selection, proposing that the long necks evolved as a secondary sexual characteristic, giving males an advantage in “necking” contests over females.
Dinosaurs did not go extinct due to being maladapted or unable to cope with change, a view found in many older textbooks. In fact, dinosaurs comprised an adaptive and successful group, whose demise was brought about by an extraordinary event that also extinguished many groups of plants, mammals and marine life. The most commonly cited cause is that of an asteroid impact on the Yucatán Peninsula, triggering the Cretaceous–Paleogene extinction event. Also, not all dinosaurs went extinct. Birds evolved from small feathered theropods in the Jurassic, and while most dinosaur lineages were cut short at the end of the Cretaceous, some birds survived. Consequently, dinosaur descendants are part of the modern fauna.
Mammals did not evolve from any modern group of reptiles. Soon after the first reptiles appeared, they split into two branches, the sauropsids and the synapsids. The line leading to mammals diverged from the line leading to modern reptilian lines (the sauropsids) about 320 million years ago, in the mid Carboniferous period. Only later (in the late Carboniferous or early Permian) did the modern reptilian groups (lepidosaurs, turtles and crocodiles) diverge. The mammals themselves, being the only survivors of the synapsid line, are the “cousins” rather than “siblings” of modern reptiles.