On the Origins of the Cells

Welcome back to the blog! As I mentioned previously, today’s post is regarding the beginning of life. Without further ado, let’s jump in to the experiment:

The Urey-Miller Experiment

The Quest for the Origins of Life

If there were life on any other planet on the universe, how similar do you think it would be to that of our Earth? Would alien lifeforms be based on Carbon as ours and have a DNA that helps them replicate? One can only speculate about such questions and provide hypotheses (of which there are millions if not billions already!). But there is yet another simple question that one can look into to, indirectly, find the answer: How did life originate on Earth?

A Simple Origin

In the 1920s, scientists Aleksandr Oparin (Russian) and J.B.S. Haldane (British) both separately proposed what is now known as the Oparin-Haldane hypothesis. It states:

Life on Earth could have arisen step-by-step from non-living matter through a process of “gradual chemical evolution”

Oparin and Haldane thought that early Earth had a reducing atmosphere (oxygen levels were low) in which molecules tend to donate electrons. Under these conditions, they suggested that:

• Simple inorganic molecules may have reacted (using energy from the sun or the occasional lightning) to create amino acids and nucleotides
• The simple organic molecules may have aggregated in the oceans forming a “primordial soup”
• The building blocks (smaller molecules) may have polymerized to form complex molecules such as proteins or nucleic acids
• Oparin suggested that colonies (aggregates) of such molecules may have initiated processes such as metabolism
• Haldane, on the other hand, thought that the macromolecules enclosed themselves in membranes and gave rise to cells

The details provided by Oparin and Haldane may not necessarily be correct. But the basic idea - stepwise, spontaneous transformation of inorganic chemicals into complex and self replicating, biomolecules - lies at the heart of most theories put forth to explain the “origin of life”.

The Experiment

In 1953, Stanley Miller and Harold Urey (he was the doctoral advisor of Miller) performed an experiment to test the Oparin-Haldane hypothesis.

Earth’s atmosphere was considered to be reducing (in the 1950s) in the pre-biotic era. As shown above, Miller and Urey tried to create a closed model of the Earth inside glass flasks and tubes. The closed system contained a heated pool of water and a mixture of gases that were thought to be abundant in early Earth atmosphere (H₂O, NH₃, CH₄, and H₂). To simulate lightning, they sent sparks of electricity through their experimental system.

After letting the experiment run for a week, the duo found various types of sugars, amino acids, lipids and some other simple organic molecules. Although no DNA or complex protein was found, the experiment showed that at least some building blocks of life could form spontaneously from simple inorganic compounds.

Why is the experiment special?

I guess most of us (scientists) now have the general knowledge to put forth a hypothesis and then conduct an experiment to test our hypothesis. The experiment seems like a very inevitable and simple one. But the very fact that the experiment was specifically designed to test a given hypothesis distinguished it from most of the experiments conducted at that time.

The Miller-Urey experiments came at a time of steeply increasing knowledge of biological macromolecules (proteins and Nucleic Acids), their structures and their functions. Notably, the experiments were published within months of other papers describing the first amino acid sequence of protein (Sanger 1953) and the elucidation of the DNA double helix by Watson and Crick (1953).

The production of amino acids (which are the basic building blocks of proteins) in the Miller-Urey experiments at conditions that might reasonably represent the early Earth provided a conceptual link between the environment from which life emerged and the types of organic compounds that could plausibly represent the first steps toward life. For perhaps the first time, it became conceivable that the initial stages in the evolution of life could be reproduced in the laboratory. Theories of the origin of life are just matters of speculation could be tested by rigorous scientific investigation.

Were the results meaningful?

The Miller-Urey experiment is probably one of the most widely recognized and influential experiments of all times. At the time of publishing, the experiment was a scientific breakthrough and has inspired numerous others to pursue the origins of life experimentally (instead of just sitting down and pondering about it like ancient Greek philosophers).

Scientists now doubt the assumption that early Earth had a reducing atmosphere. So, it’s doubtful that Miller and Urey performed an accurate simulation of the conditions on early Earth. However, a number of experiments done in the years since has shown that organic building blocks (especially amino acids) can form from inorganic molecules under a wide range of conditions.

From these experiments, it seems reasonable to imagine that at least some of life’s building blocks originated from inorganic simple molecules. The “how” (and exactly under “what” conditions) still need to be answered.