Although pakicetids were land mammals, it is clear that they are related to whales and dolphins based on a number of specializations of the ear, relating to hearing. The skull shown here displays nostrils at the front of the skull.
Anatomy (1 of 2)
Individual organisms contain, within their bodies, abundant evidence of their histories. Theexistence of these features is best explained by evolution.
Several animals, including pigs, cattle, deer, and dogs have reduced, nonfunctional digits,referred to as dewclaws. The foot of the pig has lost digit 1 completely, digits 2 and 5 have been greatly reduced, and only digits 3 and 4 support the body. Evolution best explains such vestigial features. They are the remnants of ancestors with a larger number of functional digits.
People (and apes) have chests that are broader than they are deep, with the shoulder bladesflat in back. This is because we, like apes, are descended from an ancestor who was able to suspend itself using the upper limbs.
On the other hand, monkeys and other quadrupeds have a different form of locomotion.Quadrupeds have narrow, deep chests with shoulder blades on the sides.
The cellular level All organisms are made of cells, which consist of membranes filled with water containing genetic material, proteins, lipids, carbohydrates, salts and other substances. The cells of mostliving things use sugar for fuel while producing proteins as building blocks and messengers. Notice the similarity between the typical animal and plant cells pictured below — only threestructures are unique to one or the other.
The molecular level Different species share genetic homologies as well as anatomical ones. Roundworms, for example, share 25% of their genes with humans. These genes are slightly different in eachspecies, but their striking similarites nevertheless reveal their common ancestry. In fact, the DNA code itself is a homology that links all life on Earth to a common ancestor. DNA and RNApossess a simple four-base code that provides the recipe for all living things. In some cases, if we were to transfer genetic material from the cell of one living thing to the cell of another,the recipient would follow the new instructions as if they were its own.
These characteristics of life demonstrate the fundamental sameness of all living things onEarth and serve as the basis of today's efforts at genetic engineering.
Chronology
The age of the Earth and its inhabitants has been determined through two complementary lines ofevidence: relative dating and numerical (or radiometric) dating.
Relative dating strata. As shown in the diagram, fossils found in lower strata were typically deposited first and are deemed places fossils in a temporal sequence by noting their positions in layers of rocks, known as to be older (this principle is known asbecause the layers weren't deposited horizontally to begin with, or because they have been overturned. superposition ). Sometimes this method doesn't work, either If that's the case, we can use one of three other methods to date fossil-bearing layers relative to oneanother: faunal succession, crosscutting relationships, and inclusions. By studying and comparing strata from all over the world we can learn which came first and which camenext, but we need further evidence to ascertain the specific, or numerical, ages of fossils.
Numerical dating carbon. Very old rocks must be dated using volcanic material. By dating volcanic ash layers both above and relies on the decay of radioactive elements, such as uranium, potassium, rubidium and below a fossil-bearing layer, as shown in the diagram, you can determine “older than X, but younger thanY” dates for the fossils. Sedimentary rocks less than 50,000 years old can be dated as well, using their radioactive carbon content. Geologists have assembled a geological time scale on the basis of numericaldating of rocks from around
