Cell differentiation was a critical step in the evolution of complex multicellular organisms. An early specialization was isolating cells responsible for reproduction.
Most multicellular organisms entrust the propagation of their genes to a select few germline cells amid a sea of non-reproductive somatic cells. With this the fitness of individual cells and the fitness of the entire organism become decoupled.
There are 2 basic cell types in multicellular eukaryotes: germline and soma. The fundamental difference is a reproductive division of labor.
Germline cells have the singular purpose of perpetuating the genomic lineage. Purview of all else belongs to soma. All bodily functions are accomplished by somatic cells.
Genome protection spurred the evolution of multicellular eukaryotes.
Multicellular organisms set aside germ cells to protect their genetic material, letting other cells – the soma – do the dirty work that damages DNA, their genetic building blocks. ~ American biologist Heather Goldsby
The protection essential for viable offspring is ensuring quality mitochondria. Cells can repair a lot of damage, but if the power plants sputter out the cell dies.
Mitochondrial mutations creep in slowly in plants and basal animals, so a germline isn’t needed. Plants generate gametes from pluripotent somatic stem cells.
In contrast, mitochondrial genetic errors can accumulate quickly in active animals, which have a higher metabolic rate and therefore undergo many more division cycles. Hence most bilateral animals sequester a dedicated germline early in development.
Further, mitochondrial genes are only passed to the next generation through the female germline, in the form of large eggs stuffed with thousands of mitochondria. This protects against errors, as eggs undergo fewer replication cycles than other tissue cells.
Single-sex transmission of mitochondrial genes somewhat restricts genetic variation. This is compensated for in mammals by generating far too many egg cells during development and discriminately using a relative few.
Human females are born with over 6 million egg precursor cells (ovarian follicles). 90% of these cells are selectively culled at the start of puberty in a mysterious process known as atresia.