Why do scientists believe that all species descend from shared ancestors all the way back to the earliest life form? There are multiple lines of evidence:

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1. Fossil Evidence: Fossils are evidence of prehistoric life that are recorded in the rocks. They may be bone that has been replaced by minerals, signs of burrowing through mud, etc.

 

The graphic below depicts the skeletal structure known from fossils of some of the ancestors of whales:

 

Whales are united as a group by a thick bony wall around the middle ear (tympanic bulla with involucrum) that is not found in other mammal groups. This bone walled ear is found in fossil whale species over time all the way back to land-based ancestors. (Watch the following video on whales.) Below is a depiction of different species related to whales through time, including depictions of what the fossil species above may have looked like:

 

Watch This Video on the Evolution of Whales

 

2. Genetic Evidence: The video above mentions genetic evidence of the relationship of whales to artiodactyls (even-toed ungulates). All multicellular organisms have DNA contained in the nucleus of their cells. DNA is the molecule that codes for the development and functioning of an organism from embryo to adult.

 

Depiction of DNA Structure

 

Parents pass their DNA on to their offspring through sperm and eggs. That is why offspring resemble their parents. The most closely related species are the most similar in their DNA because they share the most recent common ancestor. The less closely related species are, the more different their DNA, because it has been a long time since they last shared a common ancestor and there has been more time for differences to accumulate in their DNA.

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The animal on the planet with which humans share the most physical characteristics is the Bonobo chimpanzee. We share 98.7% of our DNA with Bonobos because we shared a common ancestor about 7 million years ago. We share about 33% of our genes with purple sea urchins. The last common ancestor we shared with purple sea urchins was over 540 million years ago.

 

Bonobo Chimpanzee and Purple Sea Urchin

 

3) Developmental Evidence: The stages embryos go through in development are very similar, for instance early human embryos have a tail and gills. They even have a tiny egg yolk, although it is not used because the baby is nourished by the mother. 

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We see multiple lines of evidence for the evolution of mammalian traits from ancestral reptilian traits:

• Reptiles lay eggs that have a yolk and amnion among other features. Developing human fetuses also have a yolk and amnion, although the yolk in humans is non-functional.  We have the same yolk genes as reptiles, but as a result of mutations over time, the genes have become non-functional. These characteristics represent developmental and genetic evidence supporting our evolutionary connections to reptiles.

• There are three major groups of mammals (organisms that have hair and nurse their young with milk): Monotremes, Marsupials, and Placentals. Monotremes, which include platypus and echidnas, lay eggs rather than having internal development of young. A connection to the mammal’s reptilian past.  Monotremes split from the line that led to Marsupials and Placentals about 166 million years ago. Marsupials split from placentals about 160 million years ago.

 

4) Morphological and Biochemical Evidence: Species share similarities in their molecules and physical structures because they inherited them form common ancestors. Tetrapods are four legged animals who trace their ancestry to a fish ancestor which colonized the land. They include amphibians, reptiles, birds, and mammals. Their limbs have a common pattern of one bone, two bones, many bones, digits (seen in the graphic below). Similarities between species as a product of shared ancestry are called homologies.

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