20.1: Introduction
It is nearly universally accepted that there was a time, however brief or long, when the earth was a lifeless planet. Given that the cell is the basic unit of life, and that to be alive is to possess all of the properties of life , any cell biology textbook would be remiss without addressing the questions of when and how the first cells appeared on our planet. Abiogenesis is the origin of life from non-living matter. Of course describing abiogenesis is no longer possible by observation! Through experiment and educated guesswork, it has been possible to construct reasonable (if sometimes conflicting) scenarios to explain the origins of life , and hence our very existence.
In this chapter, we will see that different scenarios share at least one feature, namely a set of geologic, thermodynamic and chemical conditions that favored an accumulation of organic molecules and proto-structures that would eventually become a cell. Those permissive conditions would have been an ecological, climatological, and environmental prebiotic laboratory in which many experimental cells might have formed and competed. Hence the chapter title “Origins of Life”! Multiple origins were not only possible under these conditions, but also probable! According to Jeremy England, of MIT, the laws of thermodynamics dictate that "... when a group of atoms is driven by an external source of energy (like the sun or chemical fuel) and surrounded by a heat bath (like the ocean or atmosphere), matter inexorably acquires the key physical attribute(s) associated with life”. ( Statistical Physics of Self Replication ). Here is a reminder of those key attributes, or properties of life.
Properties of Life
- Evolution : long term adaptation/speciation
- Cell-based : Cells are the fundamental unit of life
- Complexity : allows physical/biochemical changes (dynamic order)
- Homeostasis : maintains balance between change and order
- Requires Energy : needed to do work (cellular functions)
- Irritability : immediate sensitivity and response to stimuli
- Reproduction : the ability to propogate life
- Development : programmed change, most obvious in multicellular organisms
Remember, to be alive is to possess not just some, but all of these properties! If entities with all of the properties of life (i.e., cells) did originate independently, they would have reproduced to form separate populations of cells. In this scenario, less successful populations go extinct while successful ones become dominant. Successful organisms would have spread, spawning populations and generating new species. The take-home message is that if conditions on a prebiotic earth favored the formation of the ‘first cell’, then why not the formation of two, or dozens or even hundreds of ‘first cells’? However, we will see that only one successful population of cells would survive to become the source of the common ancestor of all life on earth, while other populations became extinct.
As to the question of when life began, geological and geochemical evidence suggests the presence of life on earth as early as 4.1 billion years ago. As for how life began, this remains the subject of ongoing speculation. All of the scenarios described below attempt to understand the physical, chemical and energetic conditions that might have been the ideal laboratory for prebiotic “ chemistry experiments ”. What all the scenarios share are the following requirements.
All Origins of Life Scenarios Must Explain:
- Prebiotic Synthesis of organic molecules and polymers
- the origins of catalysis and replicative biochemistry
- the sources of free energy to sustain prebiotic biochemistry
- the beginnings of metabolism sufficient for life
- the origins of molecular information storage and retrieval
- enclosure of life's chemistry by a semipermeable membrane
Let’s consider some tricky definitions. If one believes the origin of life was so unlikely that it could only have happened once (still a common view), then the very first cell (defined as the progenote , the progenitor of us all) is our common genetic ancestor.
On the other hand, what if there were many origins of life? Then there must have been more than one ‘first cell’, generating multiple populations of cells. Each such population, starting with its own ‘progenote’ would have evolved. In this scenario, only one cell population would survive; its evolved cells would have been the source of our Last Universal Common Ancestor , or LUCA . All populations of other first cells went extinct. The LUCA remains defined as the highly evolved cell(s) with genome, biochemistry and basic metabolic infrastructure that is shared among all things alive today.
Whatever the pathway to the first living cells on earth, molecular studies over the last several decades support the common ancestry of all life on earth, in the form of the LUCA. Look at the phylogenetic tree below showing the domains of life that we have seen before, with the LUCA at its root.
Regardless of the number of ‘first cells’, the LUCA’s ancestors still descended from a progenote! So, how did we get to our own progenote, or first cell? Consider these common features of any life-origins scenario:
- reduction of inorganic molecules to form organic molecules
- a source of free energy to fuel the formation of organic molecules
- a scheme for catalytic acceleration of biochemical reactions
- separation of early biochemical ‘experiments’ by a semipermeable boundary
Next, consider some proposed scenarios for the creation of organic molecules:
- import of organic molecules (or even life itself) from extraterrestrial sources
- organic molecule synthesis on an earth with a reducing atmosphere
- organic molecule synthesis on an earth with a non-reducing atmosphere
Here we explore alternate free-energy sources and pathways to the essential chemistry of life dictated by these different beginnings. Then we look at possible scenarios of chemical evolution that must have occurred before life itself. Finally, we consider how primitive (read “simpler”) biochemistries could have evolved into the present-day metabolisms shared by all existing life forms.
347 What any Life Origins Scenario Must Explain
Learning Objectives
When you have mastered the information in this chapter, you should be able to:
- Explain how organic molecules would capture chemical energy on a prebiotic earth .
- List the essential chemistries required for life and why they might have been selected during chemical evolution.
- Discuss the different fates of prebiotically synthesized organic monomers and polymers and how these fates would influence the origins of the first cells on earth.
- Compare and contrast two scenarios for extraterrestrial origins of organic molecules.
- Summarize the arguments against Oparin’s primordial soup hypothesis.
- Summarize the evidence supporting origins of life in a non-reducing earth atmosphere.
- Compare the progenote and the LUCA.
- Discuss the evidence suggesting an origin of cellular life in the late Hadean eon .
- Describe how life might have begun in deep ocean vents – compare the possibilities of life beginning in black smokers vs. white smokers .
- Argue for and against an autotroph-first scenario for cellular origins.
- Explain why some investigators place significance on the early origins of free energy storage in inorganic proton gradients .
- Define autocatalysis , co-catalysis and co-catalytic sets; provide examples.
- Define coevolution .
- Describe the significance and necessity of coevolution before life. In what ways is coevolution a feature of living things? Explain.