Last Common Ancestor
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[edit] LCA Defined
Last Common Ancestor (LCA) is the parent of a new species.
[edit] LCA of Humankind
The most famous LCA is the parent of the so-called “Missing Link.” This chimpanzee-like creature, born five or six million years ago, began the speciation that led to upright hominids and ultimately on to modern man.
Along the hominid line there have been many branches, leading, for example, to Australopithecus africanus, Homo erectus and Homo neanderthalensis. Today the term LCA is often used in reference to the branching of modern man from archaic Homo sapiens (now extinct). The Last Common Ancestor of all mankind was probably born in Africa, 100,000 - 40,000 years ago.
[edit] LCA of two species
While the new species owes all its genes to its LCA, the LCA itself will likely be a member of the original line, not the new one. Although its genetic contribution to that original line may be diminimus, as a convention the term is often used as if the LCA were the ancestor of both species.
The more recent is the Last Common Ancestor of two species the more closely related they will be. The LCA of elephant and earthworm, two vastly different species, lived two billion years ago. The LCA of elephant and tyrannosaurus lived 300 million years ago. The LCA of the elephant and mammoth, two closely related species, lived only seven million years ago.
[edit] LCA a single organism?
Narrowly defined, the LCA is the seminal individual that first coded the mutation that created the characteristic that defined the new species. However, in sexually-reproducing organisms, until the new species establishes itself there are going to be some, possibly many, generations of gene-swapping with the parent branch. Depending on the process that gives rise to the species, the LCA can often be better described as the initial small, interbreeding group or clan or founding population, all members of which carry a common species-defining mutation.
[edit] Other Common Ancestor Titles
“LUA” - The "Last Universal Ancestor" or "Last Universal Common Ancestor" (LUCA) are the names given to the hypothetical single cellular organism or single cell that gave rise to all life on Earth 3.9 to 4.1 billion years ago. This hypothesis has since been refuted on many grounds. For example, it was once that that the genetic code was universal (see: universal genetic code), but differences in the genetic code and differences in how each organism translates nucleic acid sequences into proteins, provide support that there never was any "last universal common ancestor." Back in the early 1970s, evolutionary biologists thought that a given piece of DNA specified the same protein subunit in every living thing, and that the genetic code was thus universal. Since this is something unlikely to happen by chance, it was interpreted as evidence that every organism had inherited its genetic code from a single common ancestor, aka., the "Last Universal Ancestor." In 1979, however, exceptions to the code were found in mitochondria, the tiny energy factories inside cells. Biologists subsequently found exceptions in bacteria and in the nuclei of algae and single-celled animals. It is now clear that the genetic code is not the same in all living things, and that it does not provide powerful evidence that all living things evolved on a single tree of life.[1] Further support that there is no "Last Universal Ancestor" has been provided over the years by lateral gene transfer in both prokaryote and eukaryote single cell organisms. This is why phylogenetic trees cannot be rooted, why almost all phylogenetic trees have different branching structures, particularly near the base of the tree, and why many organisms have been found with codons and sections of their DNA sequence that are unrelated to any other species.
“Common ancestor” - Any two species have millions of plain “common ancestors.” The ancestors of both the elephant and mammoth, for example, were common from as far back as three billion years ago up to their split seven million years ago - this being the date of their last common ancestor.
“Eve” - DNA studies show human mitochondria can trace its lineage to a “Mitochondria Eve” who lived possibly 150,000 years ago. It should be noted, however, that every common structure in living people - from the makings of our mitrochondria to our hair follicles - has an “Eve” that lived somewhere in the great distance between the LUA of three billion years ago and the Last Common Ancestor of possibly 40,000 years ago.
“Most Recent” Common Ancestor - The most recent common ancestors of you and your sibling are your parents. According to some accounts, Genghis Kahn is the most recent common ancestor to some thirty two million men. Computer simulations suggest that almost all people living today have some bit of DNA from an ancestor who lived possibly as few as 2,000 years ago. This person (name lost to history) has been entitled the Most Recent Common Ancestor (MRCA) of All Mankind.
The “last” and the “most recent” common ancestor of all mankind are different in two ways. First, the LCA is the founder of the species while the MRCA is simply a member of it. Second, while we all may carry some genes of the MRCA of All Mankind, we also carry genes from the millions of his contemporaries. On the other hand, every person on earth owes 100% of his genes to the last common ancestor of modern man.
[edit] Reference and other works
ActionBioscience.org - 'Origins of Modern Humans: Multiregional or Out of Africa?' Donald Johanson, American Institute of Biological Sciences (May, 2001)
N. Patterson et al. “Genetic evidence for complex speciation of humans and chimpanzees,” Nature, published online May 17, 2006. [1]
News Group Discussion sci.anthropology.paleo "Date for Last Common Ancestor?" http://groups.google.com/group/sci.anthropology.paleo/browse_frm/thread/35ade9443af17c3d/781567d4c1b1ebe6?lnk=gst&q=last+common+ancestor&rnum=2&hl=en#781567d4c1b1ebe6
BBC Science & Nature: "The Day we learned to think" http://www.bbc.co.uk/science/horizon/2003/learnthink.shtml
Richard Klein, “The Human Career”
Richard Leakey, “People of the Lake.” Suggests a recent (40,000 yrs) LCA, possibly defined by greatly improved speech capabilities.
