Common Descent
<p >In <a href="http://en.wikipedia.org/wiki/Evolutionary_biology" title="Evolutionary biology" >evolutionary biology</a>, a group of <a href="http://en.wikipedia.org/wiki/Organism" title="Organism" >organisms</a> share <b>common descent</b> if they have a common<a href="http://en.wikipedia.org/wiki/Ancestor" title="Ancestor" >ancestor</a>. There is <a href="http://en.wikipedia.org/wiki/Evidence_of_common_descent" title="Evidence of common descent" >strong quantitative support</a> for the theory that all <a href="http://en.wikipedia.org/wiki/Life_on_Earth" title="Life on Earth" class="mw-redirect" >living organisms on Earth</a>are descended from a <a href="http://en.wikipedia.org/wiki/Last_universal_ancestor" title="Last universal ancestor" >common ancestor</a>.<sup id="cite_ref-theobald_0-0" class="reference" ><a href="http://en.wikipedia.org/wiki/Common_descent#cite_note-theobald-0" >[1]</a></sup><sup id="cite_ref-steel_1-0" class="reference" ><a href="http://en.wikipedia.org/wiki/Common_descent#cite_note-steel-1" >[2]</a></sup>
<a href="http://en.wikipedia.org/wiki/Charles_Darwin" title="Charles Darwin" >Charles Darwin</a> proposed the theory of universal common descent through an <a href="http://en.wikipedia.org/wiki/Evolution" title="Evolution" >evolutionary</a>process in <i><a href="http://en.wikipedia.org/wiki/On_the_Origin_of_Species" title="On the Origin of Species" >On the Origin of Species</a></i>, twice stating the hypothesis that there was only one progenitor for all life forms and ending with "There is a grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one".<sup id="cite_ref-origin_2-0" class="reference" ><a href="http://en.wikipedia.org/wiki/Common_descent#cite_note-origin-2" >[3]</a></sup> The theory asserts that all currently living organisms on Earth share a common genetic heritage with each being the descendant from a single original species, though the suggestion of substantial<a href="http://en.wikipedia.org/wiki/Horizontal_gene_transfer" title="Horizontal gene transfer" >horizontal gene transfer</a> during early evolution has led to questions about <a href="http://en.wikipedia.org/wiki/Monophyly" title="Monophyly" >monophyly</a> of life.<sup id="cite_ref-theobald_0-1" class="reference" ><a href="http://en.wikipedia.org/wiki/Common_descent#cite_note-theobald-0" >[1]</a></sup>
The <a href="http://en.wikipedia.org/wiki/Last_universal_ancestor" title="Last universal ancestor" >last universal ancestor</a> (LUA) (also called the last universal common ancestor, LUCA), that is, the <a href="http://en.wikipedia.org/wiki/Most_recent_common_ancestor" title="Most recent common ancestor" >most recent common ancestor</a> of all currently living organisms,<sup id="cite_ref-theobald_0-2" class="reference" ><a href="http://en.wikipedia.org/wiki/Common_descent#cite_note-theobald-0" >[1]</a></sup> is believed to have appeared about <a href="http://en.wikipedia.org/wiki/Timeline_of_evolution" title="Timeline of evolution" class="mw-redirect" >3.9 billion years ago</a>.<sup id="cite_ref-3" class="reference" ><a href="http://en.wikipedia.org/wiki/Common_descent#cite_note-3" >[4]</a></sup><sup id="cite_ref-4" class="reference" ><a href="http://en.wikipedia.org/wiki/Common_descent#cite_note-4" >[5]</a></sup></p>