It is a truism to say that no one was there when life first appeared on earth.…

Concept. An argument is a set of statements in which one or more statements, called the premises, are put forward as grounds for accepting a further statement, called the conclusion. What turns a group of sentences into an argument is this inferential link — something is asserted, and something else is claimed to follow from it. Words such as thus, therefore, hence and so are conclusion indicators: they mark the point at which the passage moves from ground to claim.

Application. Split the given passage along that structure:

1. “No one was there when life first appeared on earth” is asserted as a fact about the past. Nothing is drawn from it yet, so it stands as the ground — the premise.

2. The connective “thus” announces that the next statement is being derived from the first one.

3. “Any assertion about life’s origin should be treated as a theory” is the statement that the first one is meant to support — the conclusion.

One statement is therefore offered in support of another through an explicit inference indicator. That premise–conclusion structure is exactly what the definition of an argument requires, so the two statements together constitute an argument.

Cross-check. Test the passage against the neighbouring categories:

• A historical explanation would state why some past event came about, citing its causes or conditions. The passage never explains how life originated; it defends a claim about the status any such account must be given.

• A narrative would recount a connected series of events in the order they occurred. There is no sequence of happenings here — only one factual claim and one evaluative claim drawn from it.

• A conjecture puts forward a single claim as tentative — offered for consideration rather than as something established. That label describes the standing of one claim, not the relation between two; the passage here does not merely float a claim, it presents one statement as support for another, and it is that premise-to-conclusion link, marked by “thus”, that fixes the classification.

Reading the passage as a premise plus an inferred conclusion accounts for every part of it, which is why it is classed as an argument.