Theories on life take scientists to extremes
Understanding what makes certain microbes tick will shed light on our own origins
Sunday, 5 March 2000
NEWS 10A
By Jim Erickson
THE ARIZONA DAILY STAR
SIGNS OF LIFE: First in a three-day series
About 50 miles southwest of the Sonoran port city of Guaymas, on the muddy 6,600-foot floor of the Sea of Cortez, an otherworldly community of exotic plants, animals and microbes lives in perpetual darkness, huddled around deep-sea volcanoes.
The Guaymas Basin hydrothermal vents harbor thickets of 3-foot-long red-plumed tube worms, clusters of clams and albino crabs, and colonies of black coral.
But it is the single-celled inhabitants of the scalding waters around the volcanoes that lured microbiologist Andreas Teske there in 1998.
Teske descended to the sea floor in Alvin, the three-person research submarine of the Woods Hole Oceanographic Institution.
He collected previously unknown types of microbes, and about 20 of the new strains are now growing in sealed culture tubes, bathed in near-boiling water, at his Woods Hole, Mass., lab.
The volcanic microbes belong to an ancient group of organisms so genetically distinct that it represents what microbiologists say is a separate branch on the tree of life — different from animals, plants and common bacteria. They are the Archaea, the ancient ones.
Teske and other scientists engaged in origins-of-life research say these heat-loving microbes hold clues about the dawn of life on Earth billions of years ago.
Understanding what makes them tick will shed light on our obscure origins and help inform the 21st century search for life beyond Earth.
“The chemical makeup of almost all other planets will be different from planet Earth. You cannot expect the same carefully adjusted environment of just the right amount of oxygen in the atmosphere,” Teske said.
“One has to be prepared for everything, and being prepared for everything includes a full inventory of the microbial diversity on Earth,” he said. “Because these microbes are, in their own way, already prepared for everything.
“No matter how extreme an ecological niche is, no matter how extreme a particular chemical environment, there are some microbes which are adapted to it and exploit this particular niche.”
Teske is one of several hundred researchers — biologists, chemists, planetary geologists, astronomers and others — participating in NASA’s $15 million-per-year “astrobiology” program. It’s a wide-ranging effort to trace the origins of the earliest life forms on this planet and to determine if life exists elsewhere.
Eleven U.S. astrobiology centers, including one at Arizona State University, were launched by NASA in 1998. ASU’s program received $800,000 from the space agency its first year and $950,000 for the second year.
“How did life get started here on our own planet, and how can we look for it out there. That’s what this research is all about,” said Arizona State geologist Jack Farmer, head of the Tempe astrobiology program.
“We’re trying to push things back as far as we can go, and we’re using a two-pronged approach: What can we get out of the fossil record, and what can we get out of the genetic information in the genomes of living organisms?”
Pinpointing exactly where, when and how life got started on Earth may never be possible. Geologic recycling processes have erased the earliest rock record, and genetic evidence from Earth’s first inhabitants has been blurred.
“There are some major issues in evolution that are just so difficult to track that I’m not sure I even want to think about them. One is how does life get started in the first place,” said molecular evolutionist Mitchell Sogin, head of the astrobiology team at the Marine Biological Laboratory in Woods Hole, Mass.
“Even if you could create a living system in the laboratory and evolve the kind of biological complexity that leads to cells, does that mean that’s the way life started?
“I don’t think so,” Sogin said. “It only says, this is one of the possibilities.”
Perhaps the best that can be hoped for is to keep inching closer to life’s beginning while gaining a clearer picture of the conditions that allowed it to grab a foothold here roughly 4 billion years ago.
In the process, researchers will learn where to look — and what to look for — when they search for evidence of life elsewhere in the solar system and on Earth-like planets around other stars.
Twenty-first century scientists may even be able to answer the Big Question: Is life unique to Earth, or does it spring up like mold on stale bread whenever the necessary ingredients are combined under suitable conditions?
Though no one knows for sure how life got started here, recent fossil and genetic evidence, along with the startling 1977 discovery of teeming ecosystems clustered around sea-floor volcanic vents like Guaymas Basin, have raised doubts about the venerable “primordial soup” model of life’s origins.
In 1953, biochemist Stanley Miller mixed various gases in a flask to simulate Earth’s early atmosphere, then added steam from an “ocean.”
He applied a spark to mimic lightning on the primeval planet, and voilà: A dark, tarry liquid teeming with organic compounds — the building blocks of life — formed before his eyes.
Miller’s famous experiments supported “primordial soup,” the idea that biology began in a sundrenched tidal pool billions of years ago.
But over the last 20 years, traces of microbial life have been found in some of the oldest rocks on Earth, pushing biological origins back to a hot and hazardous time when Earth’s surface was still being pelted by asteroids and comets.
There was no oxygen to breathe and no ozone layer to block harmful ultraviolet radiation.
Because exposed surface locations were so vulnerable on the early Earth, some origins researchers began looking for more sheltered environments where the three requirements for all known forms of life — liquid water, a carbon source and energy to drive chemical reactions — are present.
At the same time, new genetic techniques revealed that many of the most ancient microorganisms on the planet came from hot-water environments.
Some researchers began to suspect that life began not in some balmy Club Med beachfront lagoon, but in a hot and nasty environment such as sea-floor volcanic vents or sulfurous Yellowstone-like hot springs.
“It’s a real dichotomy, and these people hate each other,” Arizona State’s Farmer said of the primordial soup versus “hot origins” debate.
“It’s amazing how impassioned people are about their particular hypothesis.”
Through laboratory experiments, computer modeling, and studies of microbial fossils and genes, researchers on the 11 astrobiology teams are testing some of the competing origins ideas.
In his Tempe laboratory, Arizona State researcher John Holloway is exploring the idea that life originated at hydrothermal vents, undersea geysers that spew hot, mineral-rich water.
The water is heated when molten rock wells up from the Earth’s mantle at places like Guaymas Basin, where the Earth’s crust is being pulled apart to form the Sea of Cortez.
At some hydrothermal vents, dark, billowing clouds of super-hot water called black smokers shoot into the frigid sea from cone-shaped rock formations called chimneys.
Holloway and his colleagues have built a simulated black smoker in their lab.
The idea is to duplicate, as nearly as possible, the conditions at a black smoker chimney and to see if the system can synthesize organic compounds important “in the chain of events leading to life,” Holloway said.
“This is a way to investigate the hypothesis that life could have originated at black smokers, and that’s the leading idea almost everywhere in the world except La Jolla, Calif., where Stanley Miller is,” Holloway said.
Miller and others who oppose the hot-origins theory maintain that organic molecules are unstable at high temperatures and could not have combined at volcanic vents to form the first precellular proto-organisms.
Though it is still unclear where life started, all the new evidence indicates that it emerged relatively quickly, despite the inhospitable conditions on the young Earth. Many scientists now believe that life appeared within the first half billion years following the planet’s formation about 4.5 billion years ago.
That recent insight holds a lesson for researchers intrigued by the possibility that primitive life may have popped up on places like Mars and Jupiter’s moon Europa, or on planets circling distant stars.
“The lesson I take away from it is that it must not be that challenging for life to start once all the ingredients are there,” said Arizona State geochemist Laurie A. Leshin, who looks for clues to the origins of life inside meteorites.
“All the evidence points to the conditions being fairly harsh, and yet life happened,” said Leshin, a member of the ASU astrobiology team.
“So maybe that’s the answer. Maybe life happens. You put these ingredients together — which you should be doing on planets around other stars commonly — then maybe life gets started.”
In 1977, the same year the first hydrothermal vent creatures were discovered near the Galapagos Islands, another event occurred that shaped the future course of the search for life’s origins.
In November of that year, University of Illinois evolutionist Carl Woese announced the discovery of a third domain of life: bacteria-like single-celled microorganisms called the Archaea.
At the time, Woese was widely regarded as a crank for rejecting the canonical two-domain classification of biology, which separates all living things into prokaryotes and eukaryotes.
Prokaryotes – the bacteria – lack a cell nucleus. Eukaryotes – all multicellular animals and plants – have a nucleus that contains the genes.
Woese used molecular biology techniques to create the first complete family tree of all life forms, a tree that could be traced back toward its roots billions of years ago.
Clustered near the base of Woese’s evolutionary tree are the “extremophiles,” microbes that occupy hostile environments at the fringes of the habitable world.
Many of those extremists are thermophiles and hyperthermophiles, microbes that love hot and super-hot water.
The tree of life, according to Woese, is rooted in high-temperature environments.
Those findings — which remain controversial — supported a “hot origins” model and spurred renewed interest in exploring extreme environments to track life’s origins.
“This is really a beautiful case of how a major conceptual breakthrough in evolutionary theory put the emphasis on this neglected group of microbial monsters and encouraged the microbiologists to look more seriously into them, to isolate new types to see what is actually out there,” Teske said.
“And only since then have we seen how diverse and environmentally tolerant microbes actually are. It has all happened in the last 20 years,” he said.
In his Woods Hole lab, Teske grows hydrothermal vent bacteria and Archaea — including some hyperthermophiles from Guaymas Basin that thrive at temperatures from 194 to 212 degrees.
He studies vent bugs that live without oxygen, breathing sulfur instead. All of them get along just fine without sunlight.
During a 1983 trip to the Guaymas Basin, researchers discovered Methanococcus jannaschii, a methane-producing microbe. M. jannaschii was the first member of the Archaea to have all its genes sequenced, and more than half of them have no counterparts in any other organism.
The exploration of microbial diversity at Earth’s extremes constantly redefines the known limits of life.
At the same time, cataloging these extremist microbes provides scientists with an inventory they can use when looking for life beyond Earth.
Because of their metabolic diversity and adaptability, many scientists believe microbes may be much more abundant in the universe than intelligent life, which — at least on Earth — requires a narrow range of environmental conditions.
“This all feeds into our strategies for exploring the universe for life,” said ASU’s Leshin.
“Life on Earth stayed very simple for a very long time,” she said. “You have these early life forms, the bacteria, and then you look 2 billion years later and basically you still have bacteria.
“So the development of intelligence and higher creatures perhaps is more of a challenge,” she said. “Probably what we should be looking for is the simple stuff.
“If we’re always looking for little green people, we may not be so successful. Because perhaps there are a lot of bacterial worlds out there, but not necessarily a lot of highly intelligent worlds.”
