Best way to find out if element is a descendant of another

I am in the process of implementing jQuery, and taking out Prototype libraries in my codebase, and I am wondering if you could give me the best way to implement this functionality in jQuery. I am familiar with the jQuery ancestor>descendant syntax, but just want to check if an element is a descendant by true of false, like the code below: can someone give me the most efficient jQuery solution for this ?

<div id="australopithecus">
  <div id="homo-herectus">
    <div id="homo-sapiens"></div>
  </div>
</div>

$('homo-sapiens').descendantOf('australopithecus');
// -> true

$('homo-herectus').descendantOf('homo-sapiens');
// -> false

In jQuery 1.6, you can use the following code generically, e.g. targetElt and parentElt can both be DOM elements or jQuery-wrapped objects, as well as selectors:

$(targetElt).closest(parentElt).length > 0

Some of the other answers require you to refer to elements by their IDs, which isn't useful if all you have is a DOM element without an ID. Also, if you want to make sure that the targetElt is a strict descendant of parentElt (in other words, you don't want to count parentElt as its own descendant), make sure to add a targetElt != parentElt check before your call to .closest(), or use .parents().find() as Jonathan Sampson suggests.

JQuery

With jQuery >=1.4 (2010) you can use the very fast function jQuery.contains()

This static method works with DOM elements, not with jQuery elements and returns true or false.

jQuery.contains( container, descendant )

Example: To check if a element is in the document you could do this:

jQuery.contains( document.body, myElement )

Native DOM

There is also a native DOM method Node.contains() that all browsers since ie5+ supports. So you can do it without jQuery:

document.body.contains( myElement )

I would think you could take advantage of CSS style selection here, with returned length..

$('#australopithecus #homo-sapiens').length // Should be 1
$('#homo-sapiens #homo-herectus').length // Should be 0

Not exactly true/false, but checking 0/1 as a boolean should work. :)

Alternately, you could do something like $('#parent').find('#child') and check the length there.

How about

$("#homo-herectus").parents().is("#australopithecus");

You can use the is() function like so:

alert($('#homo-sapiens').is('#australopithecus *'));
// -> true

alert($('#homo-herectus').is('#homo-sapiens *'));
// -> false