Why did syntax evolve in the first place?

The adaptationist story and the by-product story make different predictions about acquisition. Only one survives contact with the data.

Framing the question

Most treatments of this rush past the setup and straight into the mechanism. That is a mistake — the framing decides which evidence counts, and getting it wrong here means every later argument inherits the same blind spot. "Why did syntax evolve in the first place?" is worth pausing on for exactly that reason.

A second, quieter issue sits underneath the first: the terms in play are used differently by different sub-fields, and papers rarely pause to reconcile them. Read charitably, most disagreements in the literature are disagreements about definitions, not about data.

What the evidence actually supports

• The core pattern is stable across the studies that report it directly.
• The exceptions cluster in one predictable place, which is itself informative.
• Where two independent methods converge, the case is strong; where only one does, treat it as a lead, not a conclusion.

The interesting cases are almost always the ones where two methods disagree.

Where this leaves us

None of this closes the question, and it shouldn't — a live research area is supposed to stay open. What it does is narrow the space of positions worth taking seriously, which is the most a single piece of writing can reasonably do.