20.12: A Grand Summary and Some Conclusions
- Page ID
- 89051
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Our consideration of how life began on Earth was intentionally placed at the end of this textbook, waiting for us to get a handle on how cells work. Clearly any understanding of life origins scenarios is very much a matter of informed if divergent speculations. Alternative notions for the origins of life entertained here all address events that presaged life under ‘best-guess’ hypothetical conditions. After trying to get a grip on prebiotic events, we asked how we got from what could have happened under a given set of prebiotic conditions to the cellular life we recognize today.
All proposals recognize that the first cells had all the properties of life (including evolution itself). Starting with that common understanding, all arguable scenarios try to navigate pathways from primitive, less controlled chemistries to more regulated and coordinated metabolisms, in other words from chemical simplicity to biochemical complexity. The chemical and metabolic evolution that began before life may have overlapped in time with cellular evolution, at least until the arrival of the LUCA.
While chemical evolution was mainly a series of selections by the physicality of a prebiotic world, the arrival of our LUCA, of life, contends with both that physical world, and with life itself. LUCA, the universal common ancestor, had already escaped the RNA world, replicating DNA, transcribing RNA, and translating mRNAs into polypeptides, all behind a semipermeable phospholipid bilayer.
Whether a heterotroph or (increasingly more likely) an autotroph, LUCA used the energy of ATP to power all of its cellular work, as do its descendants. Thus, evolution after the LUCA, is focused on continued selection of the complexities of metabolism that enables the spread and diversification of life from wherever it started. The selection of chemistries and characters encoded among by already, accumulated random neutral genetic changes is ongoing, and continues to increase the diversity of species and their spread to every conceivable ecological niche on the planet.
The take-home message of this chapter should thus be that:
- Prebiotic chemicals interacted with each in a free energy rich environment.
- Protected chemistries that resisted environmental degradation were subject to a prebiotic selection (chemical evolution).
- Chemical evolution led to a diversified chemistry set with which to experiment with combinations that could produce an entity with all the properties of life…, the first cell.
- Life has been evolving ever since!
- An understanding of the molecular basis of evolution can help us understand how life may have begun, spread, diversified, and been sustained on Earth (or elsewhere!).
Here are two final Challenge Questions to ponder:
The platypus is one of a few monotreme mammals that preceded all other mammals alive today. They are found only in Australia and Tasmania. The fact that they have 10 sex chromosomes, a duck-like bill, and lay eggs are not their only peculiarities. Define the term “missing link”, check out the link at The Peculiar Platypus and explain why you would or would not consider the platypus a missing link?
Stephen Hawking, the famous astrophysicist, worried that as artificial intelligence (AI) matures (in computers or more ‘hands–on’ devices), it/they will evolve. Here’s a link to part of a 2016 interview: Hawking on the Evolution of AI). What do you think Hawking meant by this? Or, what do you think it might mean if, as I have intimated by this discussion, evolution is the prime directive of living things? Hawking also argued against trying to contact extraterrestrial intelligence; check it out 20 at Hawking Cautions Don't talk to Extraterrestrials.