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A mammal growing from a single cell. From Hird, Dennis. 1903. An easy outline of evolution. London, Watts & Co.
A mammal growing from a single cell. From Hird, Dennis. 1903. An easy outline of evolution. London, Watts & Co.

The figure illustrates how a new life develops from a single cell, by cell division. The entire body is built in such a way.
We now know that cell division involves the duplication of the cells' DNA, such that the DNA of the daughter cells is identical.

Wait a minuteā€¦ "Each ERV is found in every cell of an organism." Are they copies from the first single cell of a new life? How do we explain he features of ERVs?
Retroviral Proviruses
ERVs
ERV Explanation
Structured for producing viruses
Same structure, but with errors
ERVs derive from non-lethal retrovirus infections in ancestors
Makes a cell produce viruses
Does not make a cell produce viruses
Fits with them deriving from faulty retroviral infections
Viral codon bias
Viral codon bias
Fits with them deriving from faulty retroviral infections
Inserted in the DNA by a retrovirus
Not inserted by a retrovirus
ERVs are inherited.
Insertion site varies from cell to cell
Always found at the same site in every cell
Because they are the result of cell duplication
Proviruses tend to target certain types of cells
ERVs are found in every type of cell
Because every type of cell is made by copying from the first cell
Not every cell of that type is infected
Each ERV is found in every cell of an organism
Because every cell is made by copying from the first cell
Provirus content varies from one insertion to another
An ERV does not vary, going from cell to cell.
Cell division is accurate compared with reverse transcription
Individuals have only few or no types of proviruses
Individuals have hundreds of thousands of ERVs
ERVs are inherited. Individuals have very many different ancestors
Only some individuals are infected
Often, the same ERVs can be found in every individual
All members of a species have common ancestors
Usually only present in one species
The same ERVs are often found in whole groups of species
All members of groups of species can have common ancestors

ERVs are explained as copies of retroviral insertions into ancestral cells, cells whose DNA has been inherited, i.e. germ-line cells - eggs or sperm cells.