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Course: MCAT > Unit 5

Lesson 1: Critical analysis and reasoning skills (CARS) practice questions

What is life?


The greatest problem of biology is understanding the divide that exists between life and matter. There seems to be an unbridgeable gulf between them, but how could life have emerged from matter if it is fundamentally different from it? The received view, today, is that life is but an extremely complex form of chemistry, which is equivalent to saying that there is no fundamental divide between them. Primordial genes and primordial proteins appeared spontaneously on the primitive Earth and gradually evolved into increasingly more complex structures, all the way up to the first cells, the basic units of life. The problem of which molecules came first has been the object of countless debates, but in a way it is a secondary issue. What really matters is that spontaneous genes and spontaneous proteins had the potential to evolve into the first cells. This however, is precisely what molecular biology does not support.
The genes and proteins of the first cells had to have biological specificity, and specific molecules cannot be formed spontaneously. They can only be manufactured by molecular machines, and their production requires entities like sequences and codes that simply do not exist in spontaneous processes. That is what really divides matter from life. All components of matter arise by spontaneous processes that do not require sequences and codes, whereas all components of life arise by manufacturing processes that do require these entities. It is the signaling of these sequences and codes, or semiosis, that makes the difference between life and matter. It is semiosis that does not exist in the inanimate world, and that is why biology is not a complex form of chemistry.
The problem of the origin of life becomes in this way the problem of understanding how the first molecular machines came into existence and started producing new types of molecules. We have seen that chemical evolution could spontaneously produce ‘bondmakers’, molecules that had the ability to stick subunits together, and we have also seen that some bondmakers could become ‘copymakers’ by sticking subunits together in the order provided by a template. The next step was the appearance of ‘codemakers’, and that is much more difficult to account for, but in principle it has the same logic and we can regard it as a natural event (ribosomes, for example, can still arise by self-assembly from their components). What really matters is that molecular machines could arise spontaneously, and once in existence they started producing molecules that cannot be formed spontaneously. More precisely, they started producing specific genes and specific proteins and that is what crossed the gulf that divides inanimate matter from life.
The genetic code was the first organic code in the history of life, but was not the only one. We have seen that other organic codes came into existence, and that they account not only for the production of new biological objects but also for the organization of these objects into higher structures and for their interactions with the external world. Semiosis, in short, was not limited to the production of specific molecules. There are at least three different types of semiosis in Nature and we find codes at all levels of life, from the world of genes and proteins all the way up to mind and language. Physics and chemistry provide of course the building blocks of life, but what ‘animates’ matter is codes, and that is why there is a deep truth in the oversimplified statement that “life is semiosis”.
Suppose scientists discover a spongy substance on the bottom of the ocean that appears to eat small fish, expel waste, reproduce, and eventually die. The spongy substance does not contain any components that require coding or sequencing. Based on the passage, the author is most likely to classify the discovery as:
Choose 1 answer: