Chapter 17: The Origins of Life

Introduction

• Age of the earth: 4.5-4.6 billion years old
• Origins of Life: <4 billion years ago
• Primordial form: the first living thing
  • Also known as the Initial Darwinian Ancestors (I.D.A)
• Cenancestor: Last Universal Common Ancestor (L.U.C.A)
  • or the Most Recent Common Ancestor (MRCA)

What is life?

• All important organisms possess both a genotype and a phenotype.
• The most important criteria that set life apart from non-life
  • the ability to store and transmit (genotype)
  • the ability to express that information (a phenotype)
• Life: if it forms a population capable of evolving by natural selection, then it is alive

What was the Primordial Form?

• The primordial should be able to perform a biological task (phenotype) and store and transmit the information (propagation) (genotype).
• The RNA world hypothesis: catalytic RNA molecules were a transitional form between nonliving matter and the earliest cells; the primordial form was an RNA-based living system.
  • RNA can serve as a repository of information (a sequence of nucleotides) and as a catalyst.

Evidence of RNA as a Primordial Form: Ribozyme

• Ribozyme: an RNA molecule that is capable of catalyzing a chemical reaction
  • Discovered by Thomas Cech and Sidney Altman in the early 1980s, they got the Nobel prize in 1989
  • Extremely important in the reaction of intron splicing and splicing of precursors of tRNAs
• RNA simultaneously processes: genotype ss 300-1000 and phenotype (catalytic)
• A ribozyme is NOT a ribosome, it is only a piece of mRNA
• Ribozyme functions include the synthesis of RNA, membranes, amino acids, ribosomes
• Ribozyme properties include catalytic behavior and naturally occurring

Evidence of the RNA World

• The most highly conserved and universal component of information-processing machinery in cells in the ribosome (tRNA, rRNA)
• Many of the cofactors that play so many roles in life are based on ribose (ATP, GTP, NAD, FAD, cAMP, coenzyme A)
• In the cell, all deoxyribonucleotides are synthesized from ribonucleotide precursors
• Many bacteria control the transcription and or translation of specific genes with “riboswitches”
• Small RNAs: post-transcriptional regulators that bind to complementary sequences in 3’ untranslated region (3’ UTRs) of target mRNAs usually resulting in gene silencing

What was the MRCA of all extant organisms?

• MRCA: most recent common ancestor
  • The most likely cellular form
• The 1st cellular life appeared > 2 billion years ago
• The advantages of a cellular membrane: Compartmentalization, accumulation of constituents, and allowing genotypes and phenotypes to be linked
• The oldest fossil of cells are 3.4 billion years old

Tree of Life

• The traditional view is the 5 kingdom view
  • this is very wrong
• Tree of life: reconstructs the phylogeny of all living things
• The challenge in using nucleotide sequence data to find a gene trait
  • shows recognizable sequence similarities
  • an essential function in all organisms (universally conserved)
  • subject to strong stabilizing selection
• So we use small subunit RNA

Based on Sequence Data

• This tree has three main branches (Bacteria, archaea, and Eucarya)
• Prokaryotes
  • Bacteria
  • Archaea: usually live in extremely harsh environments
  • more closely related to Eukaryotes than bacteria
• Protists are scattered on the eukaryotic branch

Gene Trees vs. Species Tree

• When we use genes involved in storage and processing information → Archaea and Eukarya are closely related relatives
  • consistent with the Small Subunit rRNA Tree of Life
• When we use genes involved in metabolism → Bacteria and Archaea are closely related
• With other genes → Bacteria and Eukarya are closely related

Gene Trees can be unreliable

• Gene Trees can be unreliable due to horizontal gene transfer
• Horizontal Gene Transfer: the movement of genetic material across the species barrier
• Universal phylogeny is based on a concatenated sequence of 31 universal genes
• The cenacestor was not a single species but a community