20.1: Introduction

It is nearly universally accepted that there was a time, however brief or long, when the Earth was lifeless. Given that the cell is the basic unit of life, and that to be alive is to possess all the properties of life, any cell biology textbook would be remiss without addressing at some length, 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, any observation of abiogenesis in nature is no longer possible! But a combination of experiment and educated guesswork makes it 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 require consistent assumptions about climatic, geologic, thermodynamic, and chemical conditions that favored abiogenesis. The “right” conditions would lead to a prebiotic accumulation of organic molecules, chemical reactions and proto-structures that could support the formation of a cell. One might reasonably speculate such a prebiotic laboratory would have led to spontaneous experiments in chemical evolution, which chemical combinations best survived and interacted in a hostile pre-biotic environment, well before the origin of the first cell… or cells. Hence the chapter title “Origins of Life”!

While many consider the creation of the first cell to be a singular, one-off event, it has been argued that multiple independent 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 (heat) like the ocean or atmosphere), matter inexorably acquires the key physical attribute(s) associated with life” (check out Statistical Physics of Self-Replication for more information). Here is a reminder of those key attributes, or properties of life:

\begin{aligned}
&\text{Evolution:} \quad &&\text{long-term adaptation/speciation} \\
&\text{Cell-based:} \quad &&\text{the cell is the fundamental unit of life} \\
&\text{Complexity:} \quad &&\text{dynamic order, allows physical/biochemical change} \\
&\text{Homoeostasis:} \quad &&\text{living things maintain balance between change and order} \\
&\text{Requires Energy:} \quad &&\text{needed to do word, i.e., all cellular functions} \\
&\text{Irritability:} \quad &&\text{immediate sensitivity and response to stimuli} \\
&\text{Reproduction:} \quad &&\text{sort of self-explanatory, yes?!} \\
&\text{Development:} \quad &&\text{programmed change; most obvious in multicellular organisms but found in some form in all organisms.}
\end{aligned}

Remember that to be alive is to possess not just some, but all these properties! If entities with the required properties of life (i.e., cells) did originate independently, they would have reproduced to form separate populations of cells, each of which would embark on an independent evolutionary pathway. In this scenario, less successful populations would go extinct as successful ones become dominant. Successful organisms would have spread, spawning populations, evolving, and generating new species. This could go on until all other independent lineages of life were extinct. The take-home message here is that if conditions on a prebiotic earth favored the formation of a single progenitor ‘first cell’, then why not the formation of two or dozens or even hundreds of ‘first cells? We may never know because there are no survivors of such lineages. If they ever existed, they are now extinct! We will see the evidence that there is only one successful population of cells from which common ancestor came all known living things.

As to the question of when life began, we will look at geological and geochemical evidence suggesting 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. After a brief history of thought about life origins, all the scenarios to be described below try to understand the physical, chemical, and energetic conditions that might have been the ideal laboratory for prebiotic “chemistry experiments”. We will examine two of these scenarios in some detail. What all Origins of Life scenarios must share are the following:

• prebiotic synthesis of organic molecules, monomers & polymers
• origins of catalysis & replicative chemistry
• sources of free energy to sustain prebiotic chemistry
• beginnings of metabolism sufficient for life
• origins of molecular communication (information storage and retrieval)
• enclosure of life’s chemistry behind a semipermeable boundary

Now let’s consider some tricky definitions. If one believes (as is still common), that the origin of earthly life was so unlikely that it could only have happened once, then the first cell, the progenote, is indeed the progenitor of us all, the common ancestor of all living things. The evolutionary tree in Figure 20.1 (below) puts the progenote at the root of the tree.

Figure 20.1: Descent (evolution and speciation) from a single progenitor cell formed by sheer luck on a prebiotic Earth.

The progenote, that first and only cell, would have reproduced and diverged into multiple, separate, giving rise to multiple lineages. In other words, the descendants of the progenote would have evolved from the get-go. Like any populations of cells and organisms, whether from of a one or many “progenotes”, descendant populations would have competed for resources (and ultimately survival) on an early Earth. Thus, one of these lineages (the dashed line in the illustration) would have survived as others fell to extinction, a process that continues today as the fossil record attests. Now let’s imagine multiple independent ‘first cells’. How would that change our view of life’s descent? Figure 20.2 illustrates a hypothetical scenario.

Figure 20.2: Descent (evolution and speciation) from multiple “first’ cells” assumes no progenote and the extinction of all but one lineage, whose descendants include our ‘last universal common ancestor’ (LUCA).

If we assume multiple life origins (“first cells”), there was no progenote. But, without evidence (we have none!), we must surmise that the evolution of all but one of these “first cells” led to extinction of their lineage, leaving only one path to the evolution of life on Earth, the one on the right. Descendants of the ‘first cell’ in that path survived to evolve their own lineages, among which many themselves went extinct. The surviving lineage that produced all living things traces back to the Last Universal Common Ancestor (LUCA).

In other words, whether life had one or many origins in a permissive prebiotic environment, we can anchor our assumptions on the evolution of life from a single origin. We will see clear evidence to support that all organisms alive today are descended from a single cell. In a scenario where only one cell population survives the competition with other populations, its evolved cells would have been the source of our Last Universal Common Ancestor, or LUCA. The LUCA must be that highly evolved cell whose genome, biochemistry, and basic metabolic infrastructure is shared among all things alive today (Figure 20.3).

Figure 20.3: Evolutionary (i.e., phylogenetic) tree of all living organisms showing descent of the 3 domains of life (Bacteria, Archea and Eukaryota) from a LUCA. The node separating archaea from eukaryota is the LECA, (Last Eukaryotic Common Ancestor), but could just as well be called the LACA!

This and similar evolutionary trees for known living things on Earth are based on studies of nucleotide sequences of many genes in existing organisms. Whatever the prebiotic pathway (or pathways) to the first living cells on Earth, computers can be used to trace differences in gene nucleotide sequences back to changes that would have begun in that single common ancestor that we have defined as our LUCA.

What is the basis for our conclusion that the progeny of multiple ‘first cells’ are no longer with us? Even if there were multiple life origins, each of the “first cells” might survive long enough to produce its own lineage of evolved cell populations. But the cellular biochemistries and structures of these independently lineages would likely have been quite different as they evolved their own solutions to the tasks of survival. Perhaps it will be easiest to imagine that they evolved very different genetic codes, even if they used RNA and DNA as their genetic material, and even if the best solution to storing and retrieving genetic information was the three-base codon for their amino acids. If life originated multiple times and any of their descendants survived to this day, we would expect them to have a different genetic code, if not a totally different way to store genetic information. With only evidence of a shared genetic code among today’s living things, we can conclude that but one of many early cell lineages went extinct, or that they never happened. Either way, among the descendants of our progenote is a lineage, a population that includes the first cell with a DNA genome with the same genetic code used by virtually all organisms alive today. This, our LUCA, and its descendants display the high level of conservation of gene and protein sequences that we find today. We’ll look at more supporting evidence that this is so as we move through this chapter. We’ll learn that this evidence is perhaps the strongest support for realized expectations. Let’s begin by looking at the following basic requirements 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 mechanism for anabolic metabolism, the synthesis of macromolecular polymers from organic monomers.
• a pathway to the catalytic acceleration of biochemical reactions
• separation of early biochemical experiments by a semipermeable boundary.

Then, we’ll 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.

We’ll explore alternate free energy sources and pathways to the essential chemistry of life dictated by these alternatives. Then we look at specific scenarios of prebiotic chemical evolution. Finally, we consider how primitive (read “simpler”) biochemistries could evolve into the metabolisms shared by all existing life forms today…, including a common genetic code!

347 What any Life Organs Scenario Must Explain

Learning Objectives

When you have mastered the information in this chapter, you should be able to:

1. Explain how organic molecules would capture chemical energy on a prebiotic earth.
2. List the essential chemistries required for life and why they might have been selected during chemical evolution.
3. 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.
4. Compare and contrast two scenarios for extraterrestrial origins of organic molecules.
5. Summarize the arguments against Oparin’s primordial soup hypothesis.
6. Summarize the evidence supporting origins of life in a non-reducing earth atmosphere.
7. Compare the progenote and the LUCA.
8. Discuss the evidence suggesting an origin of cellular life in the late Hadean eon.
9. Describe how life might have begun in deep ocean vents – compare the possibilities of life beginning in black smokers vs. white smokers.
10. Argue for and against an autotroph-first scenario for cellular origins.
11. Explain why some investigators place significance on the early origins of free energy storage in inorganic proton gradients.
12. Define autocatalysis, co-catalysis and co-catalytic sets; provide examples.
13. Define coevolution.
14. Describe the significance and necessity of coevolution before life. In what ways is coevolution a feature of living things? Explain.