user725: The modern rules of chess, except for the stalemating being a draw, are basically equivalent to a game in which the goal is just to capture the king, not checkmate him. It's plausible that in the oldest versions of chess, this was indeed the goal, but since a rational player would then always move their king when in check, and never leave themselves in check, and would only have their king captured when there was no escaping from it, players may have decided that responding to check may as well just be made a rule.
I haven't been able to find any reliable source either confirming or refuting this hypothesis. Is there any?

user10308: I think you may be right, but I can't find anything to support it.
However, if the goal was originally to actually capture the opponent's king, this raises another interesting question:
What happens if the goal is to capture the king, moving into check is allowed, and you still have no legal move?
For example:
   [fen "3k4/8/8/8/1p1p4/pPpPp3/PRP1P3/NKN5 w - - 0 1"]    

If it is White to move, this position is stalemate under modern rules, but if there's no stalemate, and Black has to capture the white king to end the game, does White just skip a move, and then 1…axb2 2.Kxb2, cxb2#, or would they just look at material, position, whatever, and work out who should win, agree to a draw, or just end the game with no result and replay it or something?

user291: Wikipedia claims that

In early Sanskrit chess (c. 500–700) the king could be captured and this ended the game. The Persians (c. 700–800) introduced the idea of warning that the king was under attack (announcing check in modern terminology). This was done to avoid the early and accidental end of a game. Later the Persians added the additional rule that a king could not be moved into check or left in check. As a result, the king could not be captured,[13] and checkmate was the only decisive way of ending a game.[14]

References [13] and [14] are to Davidson, Henry (1949), A Short History of Chess, which can be consulted on Google Books. I couldn't tell immediately what sources Davidson used.

725: <p>The modern rules of chess, except for the stalemating being a draw, are basically equivalent to a game in which the goal is just to capture the king, not checkmate him. It's plausible that in the oldest versions of chess, this was indeed the goal, but since a rational player would then always move their king when in check, and never leave themselves in check, and would only have their king captured when there was no escaping from it, players may have decided that responding to check may as well just be made a rule.</p>

<p>I haven't been able to find any reliable source either confirming or refuting this hypothesis. Is there any?</p>
 10308: <p>I think you may be right, but I can't find anything to support it.
However, if the goal was originally to actually capture the opponent's king, this raises another interesting question:
What happens if the goal is to capture the king, moving into check is allowed, and you <strong>still</strong> have no legal move?
For example:</p>

<pre><code>   [fen "3k4/8/8/8/1p1p4/pPpPp3/PRP1P3/NKN5 w - - 0 1"]    
</code></pre>

<p>If it is White to move, this position is stalemate under modern rules, but if there's no stalemate, and Black <strong>has</strong> to capture the white king to end the game, does White just skip a move, and then 1…axb2 2.Kxb2, cxb2#, or would they just look at material, position, whatever, and work out who should win, agree to a draw, or just end the game with no result and replay it or something?</p>
 291: <p><a href="https://en.wikipedia.org/wiki/Checkmate#History">Wikipedia</a> claims that</p>

<blockquote>
  <p>In early Sanskrit chess (c. 500–700) the king could be captured and this ended the game. The Persians (c. 700–800) introduced the idea of warning that the king was under attack (announcing check in modern terminology). This was done to avoid the early and accidental end of a game. Later the Persians added the additional rule that a king could not be moved into check or left in check. As a result, the king could not be captured,[13] and checkmate was the only decisive way of ending a game.[14]</p>
</blockquote>

<p>References [13] and [14] are to Davidson, Henry (1949), <em>A Short History of Chess</em>, which can be consulted on Google Books. I couldn't tell immediately what sources Davidson used.</p>
