What does “which” rfer to in this sentence?

Since before the fishes left the so water, the most intelligent form of life present on Earth in each era has been the rootstock out of which new and still more intelligent forms have evolved.
What does “which” rfer to, the rootstock or the most intelligent form of life present on Earth ?

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Since before the fishes left the so water, the most intelligent form of life present on Earth in each era has been the rootstock.

From that rootstock [out of which] new and still more intelligent forms have evolved.

The rootstock

Hi Vickie,

Both!

Mark...

Good day, Mark.

How can it be both? If we take out 'rootstock' and its verb,

Since before the fishes left the so water, the most intelligent form of life present on Earth in each era out of which new and still more intelligent forms have evolved,

we have an ungrammatical example.

No, I don't think it's both.

The rootstock predates the new and still more intelligent forms.

The new and still more intelligent forms evolved out of the rootstock.

It is the rootstock out of which they evolved.

The "which" refers back to the rootstock.

Excellent points, Firefly!