George's Cosmic Treasure Hunt (19 page)

THE USER'S GUIDE TO THE UNIVERSE

DID LIFE COME FROM MARS?

Where and when did life as we know it begin? Did it begin on Earth? Or could it have come from Mars?

A couple of centuries ago most people believed that humans, and other species, had been present since the creation of the Earth. The Earth was thought to be the entire material world, and the creation was described as a sudden event, like the Big Bang that scientists believe in today. This was taught in creation stories, like the one in Genesis, the first book of the Bible, and other cultures throughout the world have similar stories of a single moment of creation.

Although some astronomers did think about the vastness of space, its study only really began after Galileo (1564–1642) made one of the first ever telescopes. His discoveries showed that the Universe contained many other worlds—some of which could, like our own planet, be inhabited. The immensity of the Universe—and the evidence that its creation must have happened
long before
humans arrived on the scene—did not begin to be generally recognized until much later on, in the eighteenth century. During that innovative time the study of rock formation by sedimentation in shallow seas led geologists to understand that such processes must have been going on, not just for thousands or even millions of years, but for thousands of millions of years—what we now call gigayears.

Modern geophysicists now believe our planet Earth—and our Solar System—was formed about 4.6 gigayears ago, when the Universe—now aged about 14 gigayears—was itself just more than 9 gigayears old.

Modern humans appear to have arrived in the rest of the world from Africa fifty thousand years ago, but modern archaeology has shown quite clearly that it was only about six thousand years ago that early human societies began to develop what we call
civilization
—economic systems with the exchange of different kinds of goods. A very important factor in any civilization is the exchange not just of goods but of
information
. But how was this information stored or spread?

Before the invention of paper and ink, one of the earliest methods was to use marks scratched on clay tablets—the distant ancestors of modern computer memory chips. This sharing and collection of
knowledge, particularly the kind we now call
scientific
, became an objective in its own right.

The development of civilization depended, of course, on the emergence of what has been called
intelligent life
—beings with a sufficient sense of self-awareness to recognize themselves in a mirror. There are several known examples on our own planet: elephants, dolphins, and, of course, anthropoids—the group that includes chimpanzees and other apes, Neanderthals, and modern human beings like us. So far no signs of intelligent life have been detected elsewhere in the Universe.

How did intelligent life on Earth come into being? Fossils had suggested the idea that modern plants and animals could have arisen from other life-forms present on Earth in earlier times, but people couldn't understand how the various species could be so well adapted without having been designed in advance. The idea of continuous evolution became generally accepted only after Darwin (in 1859) explained the principle of adaptation by
natural selection
. Understanding how this actually works, however, only became possible in the late 1950s, when Watson and Crick made their discoveries about DNA.

This modern DNA-based understanding of the evolutionary process is supported by the fossil record—as far back as it is dated. The trouble is that the record does not go very far back—less than a gigayear, which is only a fraction of the total age of the Earth.

Early, simple life-forms developed before what is known as the Cambrian era. We can see fairly clearly how (though not precisely why) what we should recognize as intelligent life-forms evolved from them over the last five hundred million years. But there is no proper record of how the pre-Cambrian life-forms evolved in the first place.

One problem is that it is only since the Cambrian era that large
bony animals, which turn easily into fossils, have been present. Their largest predecessors are believed to be soft-bodied creatures (like modern jellyfish); farther back in time, the only life-forms seem to have been microscopic single-celled creatures. These don't leave clear fossil records.

Going back even farther, it is evident that evolution must have been very slow. And tricky to achieve. Even if environmentally favorable planets were fairly common in the Universe, the odds against the evolution of advanced life on any single planet would have been very high. This means that it would occur on only a very small fraction of them. Earth must be one of those rare exceptions. And it could still have easily gone wrong. There is a calculation by astrophysicists known as the
solar age coincidence
. This shows that in the time taken by evolution on Earth to lead to intelligent life, a large part of the hydrogen fuel reserves powering our Sun were used up. In a nutshell, if our evolution had been just a little bit slower, we would never have got here at all before the Sun burned itself out!

So which of the essential evolutionary steps would be the hardest to achieve in the available time?

One difficult step on Earth may have been the beginning of what is known as
eukaryotic life
—in which cells have an elaborate structure with nuclei and ribosomes. Eukaryotes include large, multicellular animals like us, as well as single-celled species like the amoeba. The fossil record shows that the first eukaryotic life appeared on Earth at the beginning of the Proterozoic aeon, about two gigayears ago, when the Earth was only about half its present age. Before this period, more primitive
procaryotic
life-forms, such as bacteria (with cells that are too small to contain nuclei), are now thought to have been widespread when the Earth was less than one gigayear old.

There is evidence for the existence of this kind of primitive life right back at the very beginning of this aeon. So we are now faced with a puzzle, because this implies that the whole process by which life actually originated must have occurred during the
preceding
epoch. This is known as the Hadean aeon, the earliest aeon of the Earth's history.

Why should this be a problem? Well, the Hadean aeon was certainly long enough—nearly a gigayear—but conditions on Earth at
that time would have been literally infernal, as the name suggests (Hades is the ancient Greek version of hell). This was when debris left over from the formation of the Solar System was crashing into the Moon and forming craters there. And the Earth—with its greater mass and gravitational attraction—would at that time have been subject to even heavier cratering. This bombardment would have caused frequent reheating of our planetary environment. New life-forms could hardly have avoided being nipped in the bud.

The planet Mars, however, has a lesser mass than Earth and is farther away from the Sun, so it has recently been proposed that the bombardment of Mars could have subsided sooner than that of the Earth. Chunks of debris may also have been frequently knocked off Mars and, in some cases, subsequently swept up by the Earth.

This would mean that life may have originated on Mars—before it could have survived here.

Analysis by electron microscope of a meteorite that did reach Earth from Mars (meteorite ALH8400) has shown structures resembling fossil microbes. This proves that
fossil
organisms may have reached the Earth from Mars. But that would still not account for
life
then appearing here unless
living
—not just fossil—organisms could survive the necessary migration by meteor. This is a question that is currently being very hotly debated.

An even more interesting question is whether the environment on Mars at that time really would have been suitable for primitive life.

Nowadays, conditions on Mars are clearly unfavorable, at least on the surface—a cold, dry desert with hardly any atmosphere except for a little carbon dioxide. Probes landing on Mars have, however, confirmed that there is a considerable amount of frozen water at the
poles. Additionally, there are many observable features of the kind expected from erosion by rivers or by surf at a seashore. This means that at some stage in the Martian past, there must have been a large amount of liquid water present—exactly what is needed for our kind of life to begin. During that early period the water would have formed an ocean. Initially this could have been several miles deep, with its center near what is now the Martian north pole.

So life could have originated at the edge of this ocean, way back in Martian history.

There are a couple of objections to this theory. One is that the atmosphere would not have contained oxygen. However, primitive life-forms on Earth are believed to have been been able to survive in an atmosphere that was also very deficient in oxygen, so that might not have mattered.

Another objection is that the ancient Martian ocean would have been too salty for known terrestrial life-forms. But maybe Martian life was originally adapted to very salty conditions, or perhaps it developed in freshwater lakes?

Thus life may well have begun on Mars—at the edge of a huge ocean there—then hitched a ride to the Earth on board a meteor. So our ultimate ancestors may, in fact, have been Martians!

 

Brandon