Very young children tend to believe that the toy car they send zooming around the house is a living thing. It moves, after all, and movement is what distinguishes an animate object from an inanimate one. In the same way, children conceive of the wind as a living thing, while a plant is not (many adults adhere to the latter view as well).
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As children grow up, their concept of life changes. Most will say that a car is not a living thing, because life requires independent movement - not movement that derives from an external source. Later, they will be able to cite other characteristics of life, such as respiration and reproduction and the capacity for death.
But what is “life” really? As the famous line about pornography goes - It may not be easy to define, but we’re pretty sure we know it when we see it.
Of course we know that a car is not a living being, nor is fire or time or a robot – but just how do we know this? What is it that differentiates between smoke and a bird, between a speck of dust and a microbe, between the inert body parts lying on a table in Dr. Frankenstein’s laboratory and a creature that suddenly raises its arm and starts to walk?
Try defining “life,” and see how long it takes before you get stuck. But trying to define life isn’t merely an intellectual game (Is a cucumber on the vine a living thing or not? What about an ear?) or a philosophical question. For an organization like NASA, this is an essential scientific query, one that’s worth a lot of money. When NASA scientists are given hundreds of millions of dollars to search for life on other planets, what exactly are they looking for? Assuming they don’t expect to find green creatures with antennae on their heads, or a cute little wrinkled fellow who keeps saying “Phone home,” what is it that a space probe might encounter that would justify the thrilling announcement that life has been discovered on another planet?
Defining “life” is also a key question for people who study its beginnings. What is the most primitive thing, structure or creature, the very first in the world, whose appearance on earth marked the start of the long-running hit show called “life?” “We know what was here before life began,” says Omer Markovitch of the Weizmann Institute of Science, who is currently completing his doctorate on the subject in the department of molecular genetics under the guidance of Prof. Doron Lancet. “We can describe how the universe was created by the Big Bang, how earth came into being and what was happening on the planet up until the stage where life could be formed: It contained a steaming soup composed of water and simple organic molecules. We also know what happened after life was formed: It developed in accordance with Darwin’s principles of natural selection and spawned all the life forms that we know today. But what happened between one thing and the other? How did we get from one moment in which the planet was entirely inanimate to the moment in which it held life? This is the great mystery that we seek to decipher.”
What gives life its vitality? Aristotle, confident in his grasp of science, spoke of three types of souls: vegetative, animal and rational, the latter of which is unique to man; Hippocrates posited the existence of three humors, each found in a different part of the body: veins, arteries and nerves; and the 17th century saw the birth of “vitalism” - a doctrine that said the processes of life are not explicable by the laws of physics and chemistry alone, and that some other aspect within them imparts vitality. This is like saying that the difference between chairs and other things is some unique aspect of the former that is found in them only and gives them their “chair-ness.” But even modern science, with its mechanistic approach that seeks to identify the physical factors at the basis of various phenomena, has been unable to formulate a satisfactory universal definition of life.
Instead, it has come up with a list of characteristics that apply to living organisms, but none of these traits can be applied to every organism across the board, or rule out all non-organisms. Take growth and reproduction, for example. Ostensibly, these apply to living things alone: All organisms grow and change form over time, in predictable fashion. They also reproduce. But chemical substances without a trace of life in them, like crystals, can also grow, and raindrops tend to subdivide. Meanwhile, a mule is an animal that cannot reproduce, and the same goes for all sterile animals. Is a barren woman not alive because she does not reproduce? And what to make of a male porcupine, which cannot reproduce without a female porcupine by its side, or a rat that is still too young to reproduce? How do they fit into the definition of life? And then there’s the extraordinary jellyfish (turritopsis nutricula) that spends its entire life passing from an adult stage to a younger stage, and back, without replicating or producing any offspring.
Another defining characteristic of life is metabolism. Simply put, living things need to eat and breathe. In order to survive, they absorb materials from outside and use them to produce energy and form other substances. Waste secretion is an integral part of life too. But this definition is also insufficient. After all, fire also consumes substances (oxygen and wood, for example), generates energy and releases waste (such as carbon monoxide); and even a gas-guzzling car generates energy from the fuel and expels the waste.
Other characteristics used to define life have also proven inadequate since they are not unique to organisms. For example, the ability to absorb and respond to stimuli - which can also be said of various devices like the ATM, the personal computer, the smoke detector; or the ability to maintain a set internal environment by the investment of energy, which is also true of engineered control systems like that of an air-conditioner, whose action is controlled by a thermostat.
In the absence of a broad general definition, a narrow definition arose stipulating that an organism’s existence must be based upon DNA or RNA molecules. This holds true for viruses, spiders, potatoes and humans – all contain these information molecules, which are not found in any of the non-living things around us. So this is an accurate definition of life, but it is so narrow that it’s akin to the definition that Adam would have given in the Garden of Eden had he been asked, What is a woman? He would likely have answered, “Eve,” but that can only be the definition of a woman when you don’t know anything else, for why not think that there is other life in the universe, that isn’t necessarily based upon DNA or RNA, but upon other molecules?
“Information is a necessary condition for life,” says Markovitch. “Because in the end, this is what enables the organism to move, to metabolize, to absorb and react to stimuli, to replicate and to be passed down to subsequent generations. But there’s another necessary condition, and that’s that this information undergo an evolution. NASA once came out with an interesting definition of ‘minimal life.’ Since NASA is searching for any sort of life on other planets, not necessarily advanced civilizations or so-called ‘intelligent life,’ it was important for them to define minimal life: what would be sufficient grounds for them to declare that life was discovered on another planet.
“I’m also interested in finding such a definition, because when I find minimal life, I’ll know that my work is done - that I’ve found the life form that may have been the first to come into being, i.e., the origin of life.
“NASA’s definition for minimal life was, ‘a self-sustaining system capable of Darwinian evolution.’ In other words: An organism must contain within it all the information it requires to sustain itself, to reproduce and to undergo evolution.”
According to this definition, viruses are not life forms.
“Correct. Viruses do undergo evolution and some - like the influenza or AIDS viruses - do it at incredible speed, and therefore manage to elude the impact of any drug, but they are not self-sustaining systems. A virus is basically a protein sac that contains very little information. It has just two or three genes sitting on a short DNA or RNA molecule, and this information is not sufficient to enable the virus to sustain itself. To eat, reproduce and evolve, the virus must invade a living creature and process the latter’s information molecules for its own benefit.”
Let’s talk a little bit about Darwinian evolution, about natural selection.
“Evolution, that is, the changing of a certain population over time, is not just an aspect of life as we know it. It’s a necessary condition for its existence. Life is only found in populations that are able to change and adapt themselves to the environment. Let’s do a little thought experiment: Let’s imagine something that is ‘the perfect replicator’: Each time it replicates it does so with 100 percent accuracy, that is, it produces offspring that are completely identical to it. Not 90 percent or even 99.9 percent, but absolutely identical.
“Since its offspring will also be perfect replicators, as will the offspring of the offspring, there is no possibility of life here. Because when the environment changes, and it will do so at some stage, and when the replicator isn’t suited to it, it and all its unsuited offspring will become extinct. That is to say that organisms must contain within themselves, within their information, the ability to change and evolve in order to adapt to the changing environmental conditions. You could say that what separates chemistry from biochemistry, the inanimate from the animate, is the history that is enfolded in the organism’s information. The information molecules were shaped by Darwinian evolution.”
But for Darwinian evolution to take place, there must be a group. There must be a number of individuals that are differentiated from one another in their information molecules. Only then can there be completion and survival of the fittest. Does this mean that the first organism was not a single individual? That the first thing that could be called “life” was a group?
“We do theorize that the first life was not a lone molecule, but rather a group of molecules that operated together and replicated as a group. This is part of our model.”
Before we get to your model - is it possible that a first form of life arose, however you wish to define it, but was unable to evolve and therefore did not survive? That doesn’t mean that it wasn’t alive.
“It’s certainly possible. Life today is the result of all that managed to survive in a changing environment. Therefore, when we’re searching for the beginning of life, we’re really looking for the minimal life that led to all the life forms we know today.”
But the definition that takes into account a system’s potential to evolve is also insufficient, because of what’s known as “artificial life.” In the early 1990s, ecologist Thomas Ray developed a computer simulation called “Tierra,” in which programs competed for vital computer resources (CPU time and access to main memory). In the process, they replicated and mutated, so that some became more effective and others less so at obtaining the needed resources. The first ones survived and “took command” of the computer until new mutations appeared that produced more complex programs that displaced the previous ones, and so on. This was the first time a successful computer simulation of an independent system fit NASA’s definition of life. These were digital creatures, composed of digital information, but they still lived in a self-sustaining and evolving system. Nonetheless, obviously no one thinks that actual life was created here.
The secret is in the membrane
So what did the first life form look like?
“In the past, the theory was that the beginning of life was a short DNA molecule. DNA is the molecule that is found in the cells of all organisms; it both contains information and can replicate, i.e, transmit itself to future generations. But in order to replicate, the DNA molecule needs the help of another molecule: a protein, whose coding information is in the DNA. And this brings us to the question of the chicken and the egg: Which came first - the DNA or the protein that replicates it? Because if the DNA molecule was first, who knew how to replicate it? And if the protein was the molecule of the beginning of life, where was the information needed to produce it located?
“Thirty years ago, an original solution to this mystery was suggested, which shifted the spotlight from the DNA molecule to the RNA molecule. Up to then, RNA molecules were thought to carry information, like DNA. And then it was discovered that RNA can also function as a protein. Scientists excitedly pounced on RNA. Here is the molecule that is both the chicken and the egg: It contains information and can also function as a protein that replicates the information. From that point on, these molecules, which offered an ‘all-in-one’ deal, became the leading candidates for the beginning of life. The standard script says that, at a certain stage, one such RNA molecule became coated with a certain covering, a membrane, that kept it distinct from its surroundings, and thus the first cell was born.”
But that’s not what you think in your lab. You propose a contrary scenario.
“We propose that the first life form was actually similar to a membrane.”
The cover rather than the book, so to speak?
“Yes. The membrane that surrounds our cells and those of all organisms is composed of fatty, or oily, molecules. We theorize that these molecules, not DNA or RNA, were the first systems that could be called life.”
And the explanation?
“We’re used to thinking of DNA and RNA as the molecules that store all the biological information, but who says this was always the case? As a chemist, I ask myself if it’s possible that chemically simpler molecules could also code information. And the answer is: Yes. Fatty molecules can also code information, and they are much simpler than DNA or RNA molecules, so they have an advantage in terms of being the first molecules of life. The fact that fat and water don’t mix also supports this idea: When you throw fat molecules into water, they bunch together spontaneously and create closed, vesicle-like structures. In other words, when we think about the primordial soup that filled the planet, it’s easy to picture how the fatty molecules that floated in it formed corpuscles, resembling hollow cells.”
How can fatty molecules code information?
“Each separate molecule does not carry information, but when they are together, the ensemble can code compositional information. Because there are multiple types of fatty molecules, each group contains a different ratio of the different types, and there are many possibilities for forming different ensembles. This is not the case with DNA and RNA molecules, in which the information is carried by a single molecule. With DNA and RNA, what produces the information are the different sequences of the four units that comprise the molecule.”
To satisfy the definition of minimal life, these fatty molecules must not only contain information but also replicate and evolve.
“That’s precisely what I’m looking at in my research - the possibility of creating in a test tube an ecological world that contains lifelike forms made of fatty molecules that can undergo evolution. The idea of the fatty molecules as the beginning of life was developed by Prof. Doron Lancet years ago. Now I’m trying to examine its feasibility by using computer simulations based on chemical and physical models.
“What happens is that the fatty molecules that bunch together create vesicle-like forms that grow over time. When a vesicle reaches critical mass, it divides into smaller sub-vesicles, and then starts to grow again. And this is where evolution comes in: It appears that the little vesicles created by the division are distinct from one another in their fat composition, and that this composition affects the growth rate of the vesicle. Certain ones cause the vesicle to grow faster and others slow growth.
“I’ve been able to show that different vesicles, distinguished by their fat composition, compete with one another for the substances in the environment. In changing environmental conditions, and due to the activity of natural selection, some survive and others become extinct. In other words, we’ve found that fat molecules are an information system that replicates itself and can undergo evolution.”
Lancet and Markovitch’s theory therefore puts the membrane before all that is contained within it. First came the simple vesicle forms that could code simple information, replicate and evolve. Later, the vesicles trapped inside themselves various amounts of the primordial soup. Until one day, one vesicle trapped within it the building blocks of RNA, which is both the chicken and the egg: information and the means to replicate it. And since it was very successful at replication, even more than the fatty molecule that surrounded it, it took over and plowed ahead by means of evolution. And so we moved from the world of fats to the world of RNA and then to the world of DNA that we know today.