Sunday, May 7, 2017

Life


"Mama, where did I come from?"
"You came from the hospital, dear."

How do we determine what is alive and what isn't? Generally, if it moves, we call "it" alive. If it breathes, if it bleeds, if it eats, if it grows... these are also indications. However, those definitions are not always correct.

Take, for instance, how crystals are formed. A crystal can grow, reach equilibrium, and even move in response to stimuli, but lacks what commonly would be thought of as a biological nervous system. We don't claim that crystals are alive. So how do we determine if something is alive or not?

Doing so is sort of like the ancient Hindu story of identifying an elephant by having each of six blind men touch only the tail, the trunk, or the leg. Each describes something different. A biologist might give a dramatically different answer from that given by a theoretical physicist.

Some agreement is possible. Living things tend to be complex and highly organized. They have the ability to take in energy from the environment and transform it for growth and reproduction. Organisms tend toward homeostasis: an equilibrium of parameters that define their internal environment. Living creatures respond, and their stimulation fosters a reaction-like motion, recoil, and in advanced forms, learning. Life is reproductive, as some kind of copying is needed for the process to repeat itself. To grow and develop, living creatures need to be consumers, since growth includes changing biomass, creating new individuals, and the shedding of waste.

To qualify as a living thing, a creature must meet some variation for all these criteria. For example, a crystal can grow, reach equilibrium, and even move in response to stimuli, but lacks what commonly would be thought of as a biological nervous system.

While a "bright line" definition is needed, the borderline cases give life's definition a distinctly gray and fuzzy quality. In hopes of restricting the working definition (at least here on Earth), all known organisms seem to share a carbon-based chemistry, depend on water, and leave behind fossils with carbon or sulfur isotopes that point to present or past metabolism.

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Life is classified among four biological families: archaea, bacteria, eukaryotes, and viruses. Archaea are the recently defined branch that often survives in extreme environments as single cells, and they share traits with both bacteria and eukaryotes. Bacteria, often referred to as prokaryotes, generally lack chlorophyll (except for cyanobacteria) and a cell nucleus, and they ferment and respire to produce energy. The eukaryotes include all organisms whose cells have a nucleus - so humans and all other animals are eukaryotes, as are plants, protists, and fungi. The final grouping includes viruses, which don't have cells at all, but fragments of DNA and RNA that parasitically reproduce when they infect a compatible host cell. These classifications clarify the grand puzzle of existing life, but do little to provide a final definition.

Attempts at definition get even more complicated when extended beyond the Earth's biosphere. The recent addition of extremophiles (archaea) to the tree of life underscores the notion that life is defined by what we know, what we have seen before, and often what we have succeeded in domesticating to a laboratory petri dish.

As revealed by its remarkable biochemical and microbiological similarities, life on Earth has a common origin. Despite this amazing morphological diversity, scientists say terrestrial life represents only a single case.

The medieval alchemists classified many different kinds of substances as water, including nitric acid (which was called "aqua fortis"). They did this because nitric acid exhibited many of the properties of water, and perhaps most importantly, it was a good solvent. It wasn't until the advent of a molecular theory that scientists could understand why nitric acid is not water. And so it goes with definitions for life.

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All theories of the origin of life face two major hurdles. The biggest is explaining the origin of the complex cooperative schema worked out between proteins and nucleic acids -- the controlled production of self-replicating catalytic systems of biomolecules. Was it God or was it random chance?

All the scenarios that have been proposed for producing RNA under plausible natural conditions lack experimental demonstration, and this includes the RNA world, clay crystals, and vesicle accounts. No one has been able to synthesize RNA without the help of protein catalysts or nucleic acid templates, and on top of that, there is the fragility of the RNA molecule to contend with.

The more serious problem, however, is the next stage of the process: the coordination of proteins and RNA through a genetic code into a self-replicating catalytic system of molecules. The probability of this happening by chance (given a random mixture of proteins and RNA) is astronomically low. Yet most researchers like to assume that if they can make sense of the independent production of proteins and RNA under natural primordial conditions, the coordination will somehow take care of itself.

The popular theory among academics postulates an initial protein world that eventually produced an RNA world as a by-product of an increasingly sophisticated metabolism. The RNA world, which starts out as an obligatory parasite of the protein world, eventually produces the cooperative schema, and hence life as we know it today. Researchers like this explanation. It's neat, it's easy, and it provides a "scientific" explanation.

Certainly, life arising from nonliving materials could occur elsewhere than Earth, but it could also have occurred on Earth. It is possible that extraterrestrial life exists and that all life nonetheless has a common ancestor. Scientists believe microbes can survive interplanetary journeys ensconced in meteors produced by asteroid impacts on planetary bodies containing life. In other words, we could all be the descendants of Martians.

It is also possible that life on Earth is the product of a very complex historical process that involves too many contingencies to be readily accessible to definitive experimental investigations. So even if we can't produce life in the lab from nonliving materials, it doesn't really follow that we will never know how life originated on Earth.

When we find other lifeforms away from Earth, it will go a long way in determining how we define what is and what is not alive. Indeed, it is exciting to consider the chances of life under the ice of one of Saturn's famous moons, or perhaps discovering and labeling microbes deep within the Earth itself, or floating freely in space.

These are big questions that will not be answered by scientists sitting in an earthly laboratory mixing chemicals and applying electric shocks to a mass of enzymes and proteins. Plus, when we consider that even science has its limitations, we begin to understand that we are bound to a materialistic consideration of these matters. In other words, if God created life, why would He share the process with us anyway?

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