By Robert Hazen, George Mason University
How did life arise? For thousands of years, humans have pondered this deep mystery, which lies at the heart of philosophy and religion and, of course, science. The answers may vary, depending on whom we ask. Yet, science proposes three possible scenarios for life’s origin. What are they?
The Three Hypotheses
The first possible scenarios for life’s origin is that life may simply have been a miracle. It may have been a divine act of intervention. If so, then the origin of life is not a scientific question. There is no experiment one can propose or an observation one can make.
Yet, it’s equally possible that the origin of life was an event that’s fully consistent with the known laws of physics and chemistry, but an extremely improbable, perhaps unique event; perhaps an event that only took place on Earth. Once again, it’s really not amenable to scientific study, because we can’t go into the laboratory and study a unique event.
And then there is a third possibility, and that’s that life is an inevitable consequence of chemistry. That, given an appropriate environment—an appropriate planet with water, for example—and sufficient time, that life always arises.
It’s only this third option that presents the prospect of scientific study. If life is likely to arise whenever and wherever appropriate conditions occur, then scientists can study life’s origin in a laboratory, through experiments. We can actually duplicate those conditions and study chemical reactions; perhaps, eventually, even create primitive life forms in the laboratory itself.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Studying the Origin of Life
Scientists learn about the origin of life in a couple of different ways. They do so by observing life on Earth today. They see the characteristics of living things, from extremely primitive single-celled organisms to very complex multicellular plants and animals. They also have an extensive fossil record, as fossils tell us about the progress, the development of life over time.
This epic history stretches back almost four billion years of Earth history. That history of life can be conveniently divided into two parts, and this defines a two-part scientific strategy for studying life’s origins and development.
The Scientific Strategy
The first period is called the era of chemical evolution: that covers the time from the Earth’s first formation up to the appearance of the first object we could call a living thing. Then, once life appeared, comes the era of biological evolution. That’s when organisms began to compete with others for a limited amount of resources, and so we have biological evolution and the process of life changing over time.
Unfortunately, it’s very difficult to know what that first life form was like, and it may have been vastly different from any sort of life that’s on Earth today. For example, it was probably not exactly like a prokaryotic cell that we see today; it was not like a simple microbe.
It’s very likely that the very first life form didn’t have DNA or RNA to pass on information. It may have had a much simpler means of reproduction and passing on information. Perhaps it didn’t even have proteins or enzymes in the way we think of them today.
Characteristics of the First Life Form
The first life form might have been an extremely thin, two-dimensional surface coating on rocks; that’s a real possibility, something that’s just a few molecules thick. This would be an invisible coating; one wouldn’t even be able to see it if they held that rock in their hand. It would just gradually spread and expand over mineral grains. But whatever that first life form looked like, it certainly had to possess three key characteristics, which are shared by all living things today.
To begin with, first of all, all life obtains energy and raw materials from its surroundings. It would have had atoms, energy, and was probably carbon-based. This is because carbon is the only element we know that’s versatile enough to produce a whole range of molecules that might lead to that first life form. Secondly, all life has the ability to grow. Surely, whether this original life form was two-dimensional or three-dimensional, whatever it was, it had to grow as it had to go from the first point of life, to expanding to fill the entire globe with life.
The final characteristic would be an ability to reproduce with variations. One can imagine a two-dimensional surface reproducing by growing and expanding, as in the case of a rock splitting in half and dividing into two surfaces.
Evolution
There could have also been a variation in a two-dimensional form, such as, a layer of organic molecules that would have been protecting it, a kind of membrane. Understandably, as there’s such a variety of carbon-based molecules, that membrane could vary from place to place in the exact kind of molecule that was included. Over time, the membranes that were most effective might have been selected during evolution.
Thus, although life may have been very different, it had to contain those three properties. It is only because of that, that we have a great variety of living things in our world today. It not only enabled the shared characteristics of cells and molecules to develop but also help in the evolution of the genetic machinery of all life.
Common Questions about the Origin of Life
Scientists learn about the origin of life by observing life on Earth today. They see the characteristics of living things, from extremely primitive single-celled organisms to very complex multicellular plants and animals. They also have an extensive fossil record, as fossils tell us about the progress, the development of life over time.
This is because carbon is the only element we know that’s versatile enough to produce a whole range of molecules that might lead to that first life form.
There could have also been a variation in a two-dimensional form, such as, a layer of organic molecules that would have been protecting it, a kind of membrane.
