Four billion years of learning has to pass through a channel narrow enough to fit in a single cell. Everything interesting about inheritance follows from how narrow that channel is.
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Evolution has had an absurd amount of time to learn.
It has run across oceans, forests, parasites, ice ages, extinctions, famines, predators, mates, wounds, births, and deaths. It has tested body plans in numbers no laboratory could afford. It has discovered wings, eyes, immune systems, flowers, teeth, milk, language-ready brains, and the strange compact miracle of a human hand.
Then all of that has to pass through a single cell.
That is the bottleneck.
A genome cannot contain a completed life. It cannot contain your future memories, your mature connectome, your language, your friends, your favourite song, your particular fear of a hallway, or the face you will one day recognize in a crowd. It cannot even specify a nervous system in microscopic detail. The channel is too narrow.
So evolution does not ship the finished model.
It ships a recipe for a learner.
The genome can send a great deal, but not everything.
It can bias development. It can shape a body. It can set up chemical gradients, growth rules, immune repertoires, hormones, drives, reflexes, sensory priorities, critical periods, and learning mechanisms. It can make some things easy to acquire and others hard. It can tune a creature toward the world its ancestors survived.
But it cannot send the world.
That is why inheritance is not duplication. A child is not a copy of a parent. A beetle is not a copy of an ancestor. The outer loop can send priors, architecture, appetites, and constraints. The inner loop must still meet reality.
This is not a flaw in inheritance. It is what makes individuality possible. If the genome could send the whole learned state, children would be continuations rather than descendants. Instead, each organism begins with shared machinery and then builds a non-shared memory layer.
The bottleneck is what prevents life from being merely replication.
It forces learning.