The idea that there was “necessarily a first reasoning being” is neither obvious nor likely.

Reasoning could very well have originally been an emergent property of a group of beings.

The animal kingdom is full of examples of groups being more intelligent than individuals, including in human animals as of today.

It’s entirely possible that reasoning emerged as a property of a group before it emerged in any individual first.

I think you are focusing too much on the fact that a being needs to be an individual organism, which is kind of an implementation detail.

What I wonder instead is whether reasoning is a property that is either there or not there, with a sharp boundary of existence.

The dead organism cannot reason. It's simply a survivorship-bias. Reasoning evolved like any other survival mechanism.

Whether the first reasoning entity is an individual organism or a group of organisms is completely irrelevant to the original point. If one were to grant that there was in fact a "first reasoning group" rather than a "first reasoning being" the original argument would remain intact.