Early Earth was a violent, nasty place when life was getting under way; it was subjected to an asteroid bombardment far worse than anything the dinosaurs ever saw

Ever since 1871, when Charles Darwin made his oft-quoted allusion to life's beginnings in a "warm little pond," scientists have tended to imagine the origin of life as being a rather tranquil affair - something like a quiet afternoon in a country kitchen, with a rich organic soup of complex carbon compounds simmering slowly in the sunlight until somehow they became living protoplasm.

Sorry, Charles. Your Warm Little Pond was a beautiful image. It's been enshrined in innumerable textbooks as the scientific theory of the origin of life. But to hear the planetary scientists talking these days, you were dead wrong. The Warm Little Pond never existed.

Earth's first billion years did indeed see the rise of bacteria, they say, including some that were advanced enough to leave recognizable microfossils. Yet during those years our world was anything but tranquil. The planet endured rampant volcanism, scorching heat, and a murderous bombardment from comets and asteroids.

So ghastly is the emerging portrait of early Earth that many origin-of-life researchers now think it warrants a complete revision of the conventional wisdom. "The field is in ferment," says Pennsylvania State University geologist James Kasting, who chaired a Cordon Conference on the origin of life this past summer, and who was co-author last year of one of the key papers dealing with the early bombardment.

The new view, admittedly controversial, is that our ancestors' birthplace had a harsh environment - like that in the sulfurous, deep-sea hot springs along the mid-ocean ridge, where magma wells up to form the spreading tectonic plates. This idea has been greatly reinforced by the realization that even the lesser asteroid and comet impacts could have vaporized the upper levels of oceans. But at the depths of the hot springs, any emerging life forms would be protected.

The bombardment pummeling earl" Earth was actually a kind of cosmic cleanup operation, explains Kasting. In the aftermath of our solar system's formation some 4.6 billion years ago, interplanetary space was still littered with megatons of construction debris in the form of rocky asteroids and icy comets roaming around at tens of kilometers per second. The resulting collisions were both inevitable and devastating, says Kasting: The newborn planets took a billion-year pounding that was several hundred thousand times more energetic than anything they've experienced since.

The scars of those early impacts have long since been obliterated on Earth by erosion and tectonic activity". But a record of that time can be seen on the battered face of the moon, whose most heavily cratered terrains are some 4 billion years old. During this period the moon was hit by at least two projectiles roughly 100 kilometers in diameter, with results that are visible today as the great lava-filled basins known as Mare Imbrium and Mare Orientale. And Earth, because of its larger size and greater gravity, had a statistical chance of getting hit by objects of a similar size (or larger) about 16 times more often.

All this had been known in a general way since the mid-1970s. But few researchers really thought very much about impacts until 1980, when the late Luis Alvarez and his collaborators suggested that a 10-kilometer asteroid or cornet may have destroyed the dinosaurs some 65 million years ago. Since then, geologists and planetary"' scientists have been thinking about impacts very hard indeed. The first paper to seriously consider what the primordial bombardment might have done to Earth was published in 1988 by California Institute of Technology geophysicists Kevin Maher and David Stevenson, who coined the term "impact frustration" to denote the repeated destruction of life forms that the impacts might have caused. Kasting, Stanford University geophysicist Nonnan Sleep, and their colleagues published an independent assessment reaching similar conclusions in 1989.

On a global scale, Kasting says, such "local" phenomena as the 1500-kilometer-wide crater and the miles-high tidal wave created by an impact would not have been terribly important. According to computer simulations, the real problem would have been the plume of vaporized rock and other debris from the impact, which would have exploded outward into space. Quickly spreading around the world, the plume would have enveloped the planet in a blanket of rock vapor having a temperature of 2000K, and a pressure about 100 times that of our modern atmosphere. Exposed to that, the oceans would have soon begun to flash into steam. In fact, says Kasting, an incoming comet or asteroid with a diameter of 400 to 500 kilometers, about the size of the modem asteroids Vesta and Pallas, would "dump in enough energy to evaporate the entire ocean."

The rock vapor would have settled back to the ground within a few months of the impact. But by then, says Kasting, the environment down below "would have long since gone from bad to worse. The vaporized ocean would have raised the surface pressure to some 270 atmospheres, and the water vapor would have acted as a greenhouse gas, trapping heat inside the atmosphere and making it very difficult for Earth to cool off. "The temperature could have easily reached 1500K," says Kasting. "So it's likely you would have sterilized the whole planetary surface." Calculations suggest that it would have taken some 2000 to 3000 years before Earth cooled enough for the first raindrops to reach the ground again, and for the ocean basins to begin to refill.

So how often did this total sterilization happen in Earth's history? It's hard to say, notes Cornell University graduate student Christopher Chyba, who has recently published some of the most careful and conservative calculations of the impact rates. The data on cratering rates is just too uncertain, especially for the earliest epochs. "Maybe Earth sustained ocean - evaporating impacts," says Chyba, "but the statistics only allow you to say it sustained zero to several of them." And each time it happened lift would have to arise anew.

But even if Earth was not struck by any of the full-scale ocean blasters, he says, there were still plenty of smaller objects whirling around the solar system say on the 100-kilometer scale. Any one of these projectiles would have stripped off the top 100 meters or so of the ocean, which is the only level where sunlight can penetrate and which is where most of the organisms in the ocean live today. As Kasting notes dryly, "That's still a sizable catastrophe." Calculations suggest that such "partial vaporizers" could have regularly sterilized the surface waters and the continents until the bombardment tapered off some 3.8 billion years ago.

For many researchers, such calculations make the idea of an origin of life at the deep-sea hot springs look very attractive. Most obviously, as Kasting and many others have pointed out, the deep-sea floor would have been relatively safe: After the last big ocean blaster, proto-organisms could have originated and flourished in the depths for hundreds of millions of years while the partial evaporators were still at work high above them.

Having that extra time, in turn, makes it easier to understand how paleontologists can find fossils of organisms that look remarkably like modern cyanobacteria (blue-green algae) in rocks that formed 3.4 to 3.5 billion years ago - only 300 to 400 million years after the bombardment. Granted, 300 million years is a long time even by geologists' standards. But bacteria, with all their machinery for cell division and metabolism, are very complicated beasts already; it's not at all clear that they could have evolved that fast in a Warm Little Pond.

Furthermore, notes University of Indiana microbiologist Norman Pace, a hot springs origin fits in well with geologists' current thinking about Earth's early volcanism. Presumably, the hot springs back then operated just as they do today, with cold water seeping down through cracks on the sea floor until it encountered a shallow magma chamber, and then roaring back upward to reemerge at temperatures as high as 4000C. The difference, says Pace, is that the primordial hot springs probably were much more widespread than today's. "The crust was so thin and so smashed up that rather than having a global network, you would have had local volcanism all over," he says. "The entire Earth's surface was like a hydrothermal vent."

Not only were the hot springs ubiquitous, adds Pace, but they would have been copious fountains of energy-rich "food stuffs"; iron ions, sulfide ions, hydrogen sulfide methane - anything that superheated water could extract from superheated rock. In the modern hot springs, these rather noxious-sounding nutrients support dense colonies of sulfur- and heat-loving bacteria, says Pace. And the bacteria, in turn, form the base of a food chain that culminates in a multitude of exotic crabs, fish, and tube worms.

Finally, as University of Washington microbiologist John Baross points out, everything we know about the hot springs suggests that they are capable of producing an immensely rich web of organic reactions - exactly what would be needed for the origin of life. As the hot water surges upward through cracks in the rock, it mixes with cold water seeping downward, thereby producing any temperature gradient and flow rate you could want. (Most of the hot springs actually emerge at no more than 450C.) The water itself is full of highly reactive metal ions such as iron, manganese, and cobalt. The rock fractures are lined with catalytic crystal surfaces and clays. It's potentially a chemical wonderland, he says.

In sum, then, the argument is that the primordial hot springs could have hosted the origin of life. But did they? One intriging bit of circumstantial evidence comes from University of Illinois microbiologist Carl Woese, who has spent more than a decade using genetic sequencing techniques to work out the evolutionary family tree of microbial lift. None of the microbes now in existence is likely to represent a common ancestor, says Woese. But of the branches that do exist, the oldest seem to be occupied by thermophiles - heat-loving bacteria. Moreover, the heat- and sulfur-loving bacteria now thriving in the hot springs belong to one of the oldest branches of all. "This suggests that the most recent common ancestor [to the modern microbial families] was thermophilic," says Woeseand - and that by extension, life itself began in an environment rich in heat and sulfur.

As it happens, however, not everyone is willing to accept that suggestion. Perhaps the strongest critic of the hot springs idea is biochemist Stanley Miller of the University of California, San Diego, whose voice carries considerable weight in the origin of life community. Back in 1953, while working as a graduate student under Nobel laureate chemist Harold Urey at the University of Chicago, Miller was the first to show experimentally that amino acids and other key ingredients for life could form spontaneously in a plausible early Earth environment.

To begin with, says Miller, he sees no problem in getting from amino acids to bacteria in 300 million years: "My position is that life had to arise very quickly, in less than 10 million years," he says. "All the prebiotic processes we know about are first."

Furthermore, he says, the hot springs are too hot. Water at 4000C would destroy organic molecules as fast as they formed. At least in a mild, cool prebiotic soup they have had a chance of surviving until they produced a more complex biochemistry.

And as for the ancestral microbes being thermophilic, he says, "I won't dispute that." But all it means is that we are descended from organisms that colonized the hot springs and survived there. He thinks it more likely that the ancestors of those thermophiles arose elsewhere under much milder conditions before being wiped out by an impact.

Washington's Baross, meanwhile, concedes that Miller has some cogent points.

Published in Science, Research News, November 23, 1990, v.250, p. 1078-1079.