Was There a Single Common Ancestor for All Life?
Lucy (the alleged human ancestor) had a distant ancestor named LUCA. That’s the assumption of many evolutionary biologists. LUCA, the Last Universal Common Ancestor, is supposed to be a single, unicellular, primitive ancestor at the root of the Darwinian tree of life .
Woese shrugs off those problems, confident a different mathematical model might be found to work. Whitfield and Woese both remind us, though, that all these difficulties and disagreements ride on top of another, more serious difficulty, even farther back in the hidden past:
1John Whitfield, “Origins of life: Born in a watery commune,” Nature 427, 674 - 676 (19 February 2004); doi:10.1038/427674a.
Lucy (the alleged human ancestor) had a distant ancestor named LUCA. That’s the assumption of many evolutionary biologists. LUCA, the Last Universal Common Ancestor, is supposed to be a single, unicellular, primitive ancestor at the root of the Darwinian tree of life .
Finding LUCA is easier said than done. Whitfield laments:“Probably all of the organic beings which have ever lived on this Earth have descended from some one primordial form,” Darwin wrote in his Origin of Species, published in 1859. Darwin had no way to peer that far back in time. But genome sequencing has given researchers hope that they can finally learn something about the ancestor of all life. In 1999, they even gave it a name, LUCA, for the last universal common ancestor.
Whitfield is not about to let frustration lead to depression. He thinks there are clues that an answer may be forthcoming. One suggested answer, however, reflects a major change in thinking about what kind of critter LUCA was:Yet despite the wealth of genomic data, LUCA has proven elusive. In theory, remnants of the organism from which all life evolved should be scattered around modern genomes. But so far, efforts to reconstruct LUCA's genes by building family trees from modern sequences have ended in frustration. Basic questions about LUCA’s nature remain unanswered. Did it live in a hot-water environment, such as a hydrothermal vent at the bottom of the ocean, or in cooler conditions at the ocean surface? Was LUCA simple, like a bacterium, or more complex?
Phylogenetic analysis (building trees from diverse genomes) presents serious statistical difficulties . Also, there is no agreement on whether LUCA was a hyperthermophile (a hot water lover) or lived near the cool ocean surface. Furthermore, only 60 “universal” genes have been found between the major kingdoms – too few by a factor of ten or more to code for a free-living organism. For these reasons, a single LUCA at the base of the tree of life is becoming increasingly difficult to accept:From all this work, one of the more surprising theories to emerge may also help to explain why LUCA has been so hard to find. Perhaps it wasn’t a single organism at all. Instead, most researchers now believe we should think of LUCA as a pool of genes shared among a host of primitive organisms.
“The naive picture that a group of organisms got all their genes from a simple last common ancestor is breaking down,” says microbiologist Gary Olsen of the University of Illinois at Urbana-Champaign. In its place, the image of a sophisticated, global community is emerging, he says. “In the past two years, it feels like it’s fallen together into a coherent picture.” Rather than a last common ancestor, LUCA may have been a last common community.
One top of that is the growing realization that horizontal gene transfer ran rampant among early unicellular organisms. To Carl Woese (U. of Illinois), that prospect is as deadly to evolutionary biology as “a fox in a hen house.” It would have scrambled the genetic record, rendering LUCA “unknowable.” That is why Woese proposed the “community” hypothesis, a world in which genes acted like modules, able to function on their own. Whitfield elaborates:According to some evolutionary biologists, the implications for LUCA are strange indeed. If a single LUCA laid the foundations for the modern diversity in membranes, metabolism and so on, it must have had several different versions of many important genes, in addition to the universal 60. Later lineages would each have pruned all but one from this set, giving rise to the current diversity in basic biochemical pathways. The idea that organisms become more complex rather than less as you get closer to the root of the tree of life is impossible to swallow, says [David] Saul [U. of Auckland, NZ]. A single LUCA “would have to have had the most bizarre biochemistry imaginable”.
Interesting in theory, but would it work? Others are not so sure it wouldn’t create bigger difficulties. One resulting problem that would have irked Darwin:Ultimately, around 3.5 billion years ago, the modern domains of life would have emerged from the gene-swapping mêlée with many of the genes from the last common community riding on their coat-tails. Inheritance and mutation would then have replaced gene transfer as the most important source of biological novelty as cells became more complex and their functions became less interchangeable. This point, says Woese, was the true origin of species, and so he has christened it the darwinian threshold.
There’s another difficulty with Woese’s idea. Mathematicians from the University of Alberta found that a gene-swapping community in a world of competing resources would have been unstable. “In other words, they say, the commune would have fallen apart.”...Patrick Forterre of the Paris-Sud University in Orsay and the Pasteur Institute in Paris, ... says the communal LUCA notion doesn’t fit with the way evolution works. “To think of LUCA in terms of a community is to remove the idea of darwinism from early evolution,” he says. Although LUCA undoubtedly swapped genes with its neighbours, Forterre argues that it would also have competed with them and ultimately triumphed through some key innovation.
Woese shrugs off those problems, confident a different mathematical model might be found to work. Whitfield and Woese both remind us, though, that all these difficulties and disagreements ride on top of another, more serious difficulty, even farther back in the hidden past:
Of course, finding LUCA would not solve the puzzle of how life began. The idea of a last common community, with a communally sophisticated biochemistry, raises another question: how did all this evolve? This is for someone else to answer, says Woese. “We don’t understand how to create novelty from scratch – that’s a question for biologists of the future.”
1John Whitfield, “Origins of life: Born in a watery commune,” Nature 427, 674 - 676 (19 February 2004); doi:10.1038/427674a.
Welcome to the biology of the future. It is called Intelligent Design. It uses well-understood principles of design detection and information theory. It can be summarized, “the essence of life is information . If the essence of life is information, the essence of information is intelligent design.”
We quoted extensively from Whitfield’s article to let you watch the Civil War going on in the Darwin Party. Both sides know they have problems, but their hypotheses each falsify one another, and neither fits the data.
If LUCA is unknowable, it is not science, it is religion . Belief in a LUCD (Living Universal Common Designer) has far fewer intellectual difficulties and fits the data like a hand in a glove.
