Introduction

It is easy to envision our earliest ancestors, hands under chins, staring out into starlit space, wondering about their existence and how they came to be. With infinite possibilities it is no wonder that a myriad of gods could show their face in the galaxies and set the stage for the birth of religions, their theology, a moral code of ethics, a special and exclusive position for humans, and eventually a method of inquiry that has led to modern science. Furthermore, the early history of thought about life elsewhere in the universe was linked to more of a theological perception of man's relationship to gods, and Earth as the exclusive domain of life – exemplified in the thirteenth-century Aristotelian–Thomistic synthesis of science and religion.

It is particularly interesting and germane to astrobiology today, that it was astronomers, and particularly those in the fifteenth to seventeenth centuries, that were the first to make quantitative observations that questioned the orthodox view of man's place in the universe. One looked to the heavens for answers to the most profound philosophical questions and in doing so helped establish the scientific method as another method of inquiry. Perhaps a lesson from this early philosophical-based history is that the drive to grow knowledge is a fundamental and perhaps evolutionary characteristic of humans. The implication is that our survival is likely to be dependent on our exploring all possibilities that help us to understand the how, why, and uniqueness of our existence. Perhaps, no topic inspires this need to explore more than the search for life elsewhere with its implications for the origin of life and the possibility of life forms unrelated to Earth life.

Introduction

In 2010, the modern scientific search for extraterrestrial intelligence was 50 years old. Its birth was marked by Frank Drake's Project Ozma using the 85-foot diameter Tatel radio telescope at the National Radio Astronomy Observatory in Green Bank, West Virginia (Drake 1961). Project Ozma observed two nearby, Sun-like stars (Tau Ceti, 12 light-years away, and Epsilon Eridani, 10.5 light-years away) for a few hundred hours and concentrated on the precise frequency at which neutral hydrogen atoms emit radiation. Since hydrogen is the most abundant element in the galaxy, Drake assumed it would be an obvious choice for any distant engineer to use for deliberate transmission (plus he could assure the observatory's management that the equipment he built, and the money he spent, could be repurposed for traditional scientific research). After 50 years of searches at optical and radio wavelengths, the results are the same as those from Project Ozma; no signals of unambiguous extraterrestrial origin have been found, though many signals from our own technologies have confused us.

Should we give up? Conclude that we are alone? Not yet. Though 50 years seems like a long time, it's the blink of an eye to a universe that is 13.7 billion years old, or to our Milky Way Galaxy that is 10 billion years old, or even to our star the Sun that is 4.5 billion years old. The cosmos is not only old, it is vast – we've hardly begun our search. Although not everyone agrees on just how large the task of searching for ET actually is, a pretty good numerical analogy comes from trying to answer a parallel question “Are there any fish in the oceans?” by scooping an 8-ounce glass of water from some ocean and examining it to see if any fish were collected. You might have caught a fish; that experiment could have worked. But if it didn't, you're not likely to be tempted to conclude that there are no fish in the oceans. Exactly the same thing holds for SETI. More searching is required; as noted in the last sentence of the first paper on SETI published in the journal Nature in 1959, authored by Guiseppe Cocconi and Philip Morrison: “The probability of success is difficult to estimate, but if we never search, the chance of success is zero.”

The asteroid streaks across the sky like a fiery messenger from the gods. As the sonic boom hits, small lizards scurry for cover, but some of the larger dinosaurs barely look up from their grazing. Seconds later, the asteroid heads out of the atmosphere into deep space, like a stone skipping off the surface of a pond.

It's been a close call. The incoming rock was the size of a large mountain. If it had been traveling a bit faster, or if the Earth had traveled slightly less far in its orbit, the asteroid would have hit head-on, causing utter devastation. Large creatures would have been killed almost instantly, and debris flung up into the atmosphere would have dimmed the Sun and disrupted the food chain, killing many more species. Instead, the Earth and its inhabitants shrug and continue their day.

Sixty-five million years pass. The dinosaurs diversify and continue their eons of dominance. Some learn to hunt cooperatively and invent simple social structures. Others develop metabolisms to deal with extremes of climate, and they expand their range on the planet.

The young scholar clutches the book to his chest as he works his way through the crowd. Campo dei Fiori is packed; it's a jubilee year, and Rome teems with pilgrims, beggars, and pickpockets. He edges forward, brushing aside the vendors who tug at his sleeve. Days earlier, a small item in a local broadsheet caught his eye. A Dominican monk from Nola was to be put to death, having exhausted the patience and goodwill of the authorities. The scholar sighs. His heart is heavy at the prospect. It is not yet a century since the death of Leonardo, but enlightenment has dimmed so much that it seems like eons.

With difficulty, the scholar climbs scaffolding behind a merchant stall so he can see over the heads of the mob. Yelling at the far side of the square tells him that Bruno has arrived, having been paraded naked through the streets of Rome. He is bound to the stake with thick rope while a local functionary reads the charges. The scholar can only catch fragments: “impenitent heretic … failure to recant … persistent follies.”

If Earth's future space travelers came to this place, they would declare it a godforsaken wilderness. A trackless vista of cliffs and escarpments stretches to the horizon. Brown rock scorches under twin giant suns in a blood-red sky. The air is a thin gruel of sulfur dioxide, methane, and nitrogen. But there's more here than meets the eye. In the pore space of the rocks, something is stirring.

At a boundary layer with the deep mantle, mildly acidic water bubbles through the rock, driven by heat and pressure from the interior. Miles underground, microbes thrive in a rich brew of organics and dissolved minerals. They move by sensing magnetic fields and temperature gradients. Huge colonies begin to differentiate their functions and metabolisms to better use the available resources. Symbiotic behavior emerges.

In the course of ceaseless and random genetic variation, some organisms develop the ability to vibrate their outer membranes and sense when it is perturbed. The timing of a return ultrasound wave acts as a primitive proximity sensor. This brand of microbes maintains the spacing to garner more resources, so it rapidly dominates the colony.

As the strategy becomes more successful, the organisms with the most powerful emission or most sensitive reception must deal with a cacophony of ultrasound signals.

In the dream, you are in an ice cave. It is starkly beautiful, suffused in blue light from an outside source. There's nothing to eat, no sustenance, just the angular planes of ice crystals. It is stunningly cold, well below freezing. Your breath billows in front of you; perspiration forms a frozen rind on your neck. You can't stay here long. Then you notice creatures working industriously along the far wall of the cave. They're oblivious to the intense cold. From the strange smell, you guess that they have antifreeze running through their veins. This place is clearly their home.

Then you awake—not to your bed but to another strange world. You are on the shores of a river, with canyon walls that rise up and disappear in the gloom. The river is acrid and filled with the worst kind of industrial effluent. The water is so acidic that it sizzles as it passes over the rocks, which are themselves discolored by chemical residue. The smell is foul and metallic, and it almost makes you gag.