In this season of Marsomania, the age-old hunt for signs of life on thered planet is relying on a potato-sized meteorite from Mars, whichmay harbor sub-microscopic fossil traces of once-living organisms.
What about proto-life here on Earth?
What if, half a billion years ago, an asteroid collided with our ownplanet, and splashed up molten meteoric material into outer space?What if this landed on some heavenly body inhabited by beings atleast as smart as Homo sapiens? And what if they discerned in thisterrestrial meteorite sub-microscopic traces of once-living organisms?
That in fact describes the situation here on Earth beginning about600,001,996 B.C., when the Cambrian geologic era succeeded thePre-Cambrian.
The Pre-Cambrian goes back to the origins of life, about 4.7 billionyears ago. Life took its time. Cyanobacteria and blue-green algaewere its only inhabitants at first, and spent the last 3.5 billion Pre-Cambrian years freeing up molecular oxygen.
"Three billion years ago," said molecular evolutionist Susumu Ohno,"the ocean was like soda pop. Its waters," he explained, "were filledwith carbon dioxide [CO2]," dissolved in the form of sodiumcarbonate [Na2CO3]."
Ohno is an emeritus scientist at the City of Hope National MedicalCenter's Beckman Research Institute in Duarte, Calif. A member ofthe National Academy of Sciences, he has an article in theAcademy's current Proceedings (PNAS) dated Aug. 6, 1996. Its title:"The notion of the Cambrian pananimalia genome."
Next week in Budapest, Hungary, Ohno will deliver an address onthis subject to an international meeting on Systemics and Evolution.
"It took the toiling cyanobacteria and blue-green algae nearly threebillion years to accumulate sufficient molecular oxygen," Ohno toldBioWorld Today, "to set the stage for animal life as oxygenconsumers, at the ocean bottom."
Paleontologists call that sudden pan-proliferation of life forms "theCambrian explosion." "Nearly every extant phylum of the kingdomAnimalia," Ohno pointed out, "emerged in a mere six to ten millionyears, early in the 600-million-year Cambrian era."
DNA Mutation _ The Constant Time Keeper
Driving this high-pressure evolution, he observed, was _ and still is_ the propensity of DNA to mutate spontaneously at a constant rate."If not interfered with by natural selection," Ohno said, "that's to say,all the mutations are allowed to accumulate, that rate amounts to 10-9[one-in-a-billion] base-pair changes per year."
You can set your evolutionary watch by this constant, Ohno implied."That's a very surprising thing," he observed. "If a man lives 68years, a bacterium only a day or two, you'd think the bacteria wouldbe changing much faster. The surprising thing is, they don't. Theyadhere to the DNA mutation rate."
In other words, "only one percent of a DNA base sequence wouldhave changed during those ten million early-Cambrian years." Fromwhich observation Ohno deduces his pananimalia "notion:"
That "blink-of-an-eye" 10 million years of the Cambrian explosion,Ohno said, "can't possibly be explained by mutational divergence ofindividual gene functions. Rather, it is more likely that all the animalswere endowed with nearly the identical genome, with enormousmorphological diversities due to differential usage of the identical setof genes."
He proposed, "that all those diverse animals of the early Cambrianperiod, some 550 million years ago, were endowed with nearlyidentical genomes, with differential usage of the same sets of genesaccounting for the extreme diversities of body forms."
Ohno nominated four gene sequences as "the more pertinent"progenitors of morphology in all modern forms of life, prokaryotic oreukaryotic. "Since every animal seems to have the same genome," hesaid, "if you find a gene in mammals or humans, you are likely to findit in sea urchins too."
His four all-purpose founder gene sequences are:
* A gene for lysyl oxidase, which regulates copper ions andmolecular oxygen, and oxidizes collagen residues: "It crosslinkedcollagen triple helices to produce ligaments and tendons, thuscontributing to the stout bodies of the Cambrian animals."
* Hemoglobin genes, transporters of molecular oxygen: "Today seensporadically in members of diverse animal phyla."
* The Pax-6 gene for eye formation: "From a marine ribbon worm toa human, eyes are organized by this gene. In animals without eyes,Pax-6 organizes other sensory systems and organs." He observed thatknockout rats deprived of Pax-6 "became eyeless and noseless aswell."
* Hox genes for the anterior-posterior (head-to-tail) body plans: "Hoxgenes are also present in all phyla of the kingdom Animalia."
Interchangeable Genes For Therapy
Besides its primary purpose of contributing to the study of evolution,Ohno suggested that his hypothetical system might find utility incertain therapeutic applications. "For example," he offered, "if youhave no Pax-6 genes, you have no eyes." He cited the congenitalophthalmological conditions or aniridia and irideremia _ absent orrudimentary irises.
To remedy this, you might bring that gene not from humans, but froma fruit fly or shrimp _ because it's the same gene."
By similar tokens, Ehler-Danlos and occipital horn syndromes aregenetic disorders in human patients involving connective-tissue andskin abnormalities, linked to the lysyl oxidase genes he describes.
At the level of evolution per se, Ohno cited recent findings thatcorrect a long-standing error as to the origin of vertebrates: "Ourwhole phylum," he noted, "is called `chordata.' We used to think thatthe first such life form was called `urochordata', the next,`cephalochordata'; then `amphioxus', and finally the fish. But now,"he said, "based on fossil data from China, we find that they all cameup at the same time, not in succession. So one cannot be the ancestorof the other."
Ohno is not a field paleontologist; he bases his proposal on meta-analysis of reports in the literature. He now is pushing the envelopeof his Animalia deduction to "the question of when the prokaryoticcyanobacteria and eukaryotic algae evolved. "It now is thought tohave been only two billion years ago," he observed, "but I think it'smuch earlier." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.