Placentas probably first evolved about 150 million to 200 million years ago. The protein syncytin, which makes up the physical barrier between mother and baby in the syncytiotrophoblast, has a certain RNA signature in its genome that has led to the hypothesis that it originated from an ancient retrovirus:
The theory is that the placenta was a virus survival mechanism. But it leads me to sinister thought about our white cell over lords. Whether can they induce random mutations in our haploid system, where sperm and egg live. A white cell mediated process sneaks a protein encoding sequence, and it gets passed along.
If evolution took a short cut. The animal in stress responds with white cell adaptions eventually infecting the haploids. So, consider the gain from walking on two feet, it comes at a cost for the more comfortable four foot animal. The white cell system can mediate the genetic selection, take the short path and skip generations of trial and error. Insert the antidote to back pain at the haploid stage and be done with it.
When human migrated north white skin solved a vitamin D deficiency. Who dunnit? Dark ski reduces vitamin D, results in skin inflammation, and the T cells respond, expressing the aggravated protein. That t cell invades the haploid gamete system and a suppressor to skin pigmentation is installed.
Evolution started with simple haploid colonies, undifferentiated cells, in the pond with various viruses. The virus successfully invaded and got a semi open system with cell differentiation inside. The random variations of genetics are not quite as random as we think.
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