Earth's History, Time Scales, and Origin of Life — Comprehensive Notes

Announcements and course logistics

  • Yesterday you were emailed about a research study in UB College of Arts and Sciences designed to support, retain, and increase achievement of undergraduate students in STEM by providing digital learning materials.
    • Participation is online and involves a questionnaire at the beginning and end of the semester, plus a mid-semester digital learning activity.
    • All materials (consent form, questionnaires, learning activities) are available via links on the UB Learn Brightspace page.
    • Participation requires about two total hours of your time over the semester.
    • You can earn up to six points of extra credit (one for each stage of the process).
    • The consent form must be completed for the first extra credit point; completion yields credit regardless of consent decision.
    • Deadline for consent form questionnaire: 5 PM on Friday, September 5, i.e., by the end of next Friday’s class.
    • For questions about the study, contact the instructor via email.
  • Meet-and-greet with the TAs this Friday after class: first 60 students receive free ice cream; opportunity to ask questions and get study tips.
  • Office hours begin this week and do not require an appointment—come by.
  • iClicker reminders:
    • You should log in when prompted during class; attendance is recorded automatically, but the instructor will not review iClicker attendance—only iClicker points.
    • If you have trouble logging in, use the support link on the basics or lecture UB Learn site; resolve before Friday for the next practice session.
    • A practice iClicker poll will be conducted; you’ll have a short window to discuss with neighbors before answering.
    • If you’re having login trouble, try to resolve it before the Friday session; a subsequent practice poll will be available.

Overview: Evolution, life, and Earth history

  • This entire course centers on evolution: how life changes over time, and what is necessary for life to form.
  • A key topic is the origin of life and what made Earth a suitable place for life.
  • Time scales and the history of the Earth will be the focus of the first part of the lecture.

Solar system formation and early Earth conditions

  • The Earth forms as part of the birth of the solar system, which itself is after the birth of the universe.
  • Before the solar system formed, a protoplanetary disk existed: a large rotating cloud of dust, rocks, and gas.
    • The disk contained hydrogen and helium formed in the Big Bang, plus heavier elements ejected by supernovae.
  • About 4.6×1094.6\times 10^{9} years ago, a nearby star exploded as a supernova, sending a shock through the protoplanetary disk.
    • The shock compressed mass toward the center, heating up and forming the Sun in the middle of the disk.
    • Perturbations from collisions allowed protoplanets to form; one such protoplanet ~1.50×1081.50\times 10^{8} km from the Sun would become Earth.
  • Earth formed about 4.57\times 10^{9}yearsagoandwaslargelycompletedwithinaboutyears ago and was largely completed within about1-\,2\times 10^{7} years after that point.
  • As the protoplanetary disk evolved, enormous heat at the center led to hydrogen fusion into helium, driving solar energy.
  • Gravity condensed matter around perturbed objects, forming rings and larger bodies that would become planets; the early Earth was far from the present state.

Precambrian Supereon: Hadean, Archean, and Proterozoic

  • The Precambrian Supereon spans most of Earth's history and is divided into three eons: Hadean, Archaean, and Proterozoic.
  • The two most important points for this course:
    • Hadean Eon (the Hell Eon):
    • Early Earth had no oceans and no oxygen in the atmosphere.
    • The planet was fully molten; early atmosphere consisted of gases from the solar nebula, plus volcanic outgassing, but no protective ozone.
    • End of the Hadean (~4.2\times 10^{9}toto3.8\times 10^{9}\,years ago) marks the cooling sufficient to begin crust formation.
    • The Earth’s surface cooled enough for crust to form, and water vapor from volcanoes and surface condensation produced oceans as rain formed in clouds.
    • Archean Eon: first life forms appear.
    • First prokaryotic fossils dated to about 3.0\times 10^{9}toto2.5\times 10^{9}\,years ago (late Archean).
    • Stromatolites (cyanobacterial mats) are common in Archean fossil records; fossils also appear in silica-rich rocks and include representatives of the domains Bacteria and Archaea.
    • By the end of the Archean, unicellular life dominated; multicellular life would not appear until later.
    • Proterozoic Eon: marks the later time when multicellular life begins to appear (late Precambrian).
    • Approximately 1\times 10^{9} years ago, the first multicellular organisms appear, transforming the biological landscape.

The Cambrian Period and the Paleozoic Era

  • The Paleozoic Era runs roughly from 5.43\times 10^{8}yearsagotoyears ago to2.50\times 10^{8} years ago; it follows the Precambrian Supereon.
  • Key developments in the Paleozoic:
    • Early Paleozoic fossils include trilobites (well-known) and marine Archaeocyathids (reef-builders similar to sponges/corals).
    • The Cambrian explosion (early Paleozoic) was once thought to be an abrupt “bang” where many animal phyla appeared within a few million years; more recent evidence suggests pre-Cambrian animals existed earlier, and the explosion may have lasted around 6\times 10^{7} years, not just a few million.
    • The origin and diversification of hard-shelled organisms increased the fossil record during the Paleozoic.
    • End of the Early Paleozoic: first land plants appear; a major milestone around 4.50\times 10^{8} years ago; possibly moss-like carpet moss on land.
    • Late Paleozoic: evolution of fish that develop legs, enabling the earliest land vertebrates; land-dwelling amphibians flourish.
    • Permian extinction at the end of the Paleozoic (the Great Dying) around 2.52\times 10^{8} years ago led to a massive loss:
    • About 96\% of all life went extinct.
    • Up to 70\% of terrestrial vertebrate species went extinct.
    • This is the largest mass extinction in Earth’s history and cleared ecological space for new life.

The Mesozoic Era: Age of Reptiles and the rise of mammals

  • The Mesozoic Era spans from about 2.50\times 10^{8}yearsagotoyears ago to6.5\times 10^{7} years ago.
  • It’s known as the Age of Reptiles; dinosaurs rise and dominate for much of this era.
  • The first mammals arise during the Mesozoic (roughly around 2\times 10^{8} years ago), representing early mammalian ancestors such as Morganucodon watsoni.
  • The Mesozoic ends with a dramatic mass extinction event known as the KT extinction (~6.5\times 10^{7} years ago).
    • KT boundary is a distinctive sediment layer found around the world.
    • The leading theory links this extinction to a catastrophic asteroid impact, accompanied by increased volcanism.
    • Dust and aerosols from these events reduced sunlight and disrupted photosynthesis, contributing to ecological collapse.

The Cenozoic Era: Age of Mammals and the rise of humans

  • The Cenozoic Era extends from the KT boundary (~6.5\times 10^{7} years ago) to the present.
  • It is often called the Age of Mammals; mammals diversify and become dominant after the KT extinction.
  • The origin of the first humans occurred a relatively recent event: about 2\times 10^{6} years ago.
  • Throughout the Cenozoic, major processes include continued continental drift, climate change, oxygen level fluctuations, and evolving biodiversity.
  • The lecturer notes a focus on diversity in this course in the latter part, with ongoing exploration of the tree of life.

Time scales visualized: geological clock and plate tectonics

  • A geological clock helps visualize Earth’s history in a circular timeline; the entire timeline is mapped onto a clock face.
    • Earth forms at the 12 o’clock position; the entire history spans a full rotation.
    • Precambrian is the longest segment, occupying most of the clock before the appearance of dinosaurs.
    • Dinosaurs appear very late on the clock (roughly around 11:58), illustrating the vastness of Precambrian time.
  • The lecturer emphasizes that the timeline is not meant for memorization of exact dates, but to understand durations and relationships among time periods.
  • Continental movement and climate shifts continually reshaped Earth’s environment and the distribution of life.
  • A link to a short movie about plate movements was provided for better intuition.

Origins of life: panspermia and abiogenesis

  • A key question is whether life began on Earth or was seeded from elsewhere (panspermia).
    • Panspermia posits life’s origins outside Earth and subsequent seeding; the lecturer notes this as one possible scenario but emphasizes it is not the central question for this course.
  • A basic working definition of life (as a working framework for discussion):
    • Life is fundamentally organized (recognizable, repeatable form) and capable of replication.
    • Additional features often debated include sensitivity, growth, regulation, and homeostasis; these are not required for the minimal definition but are commonly discussed.
  • Evidence suggests life arose around about 3.8\times 10^{9} years ago, in a world where the solid portions of Earth were limited to molten rock, oceans were not yet widespread, and there was no free oxygen in the atmosphere.
  • The Earth’s early atmosphere is characterized as reducing (electron-adding), which facilitates the formation of complex organic molecules.
    • Oxygen-rich atmospheres are oxidizing and can break bonds, whereas reducing environments enable the formation of organic compounds.
    • The atmosphere lacked an ozone layer (no UV shielding), permitting high UV flux that provided energy for chemical reactions.
  • The four proposed steps for the origin of life (abiogenesis) are as follows: 1) Abiotic synthesis of organic molecules (building blocks like amino acids, nucleotides, sugars, lipids, ATP).
    • The Miller–Urey experiments (1953) attempted to simulate early Earth conditions by applying electric sparks to a reducing gas mixture.
    • Early experiments assumed an early atmosphere of ammonia and methane, which differs from later evidence suggesting CO$2$- and N$2$-dominated atmospheres; nonetheless, the experiments demonstrated the potential to synthesize key biomolecules.
    • Modern prebiotic chemistry expands on these ideas, showing that amino acids, nucleotides, sugars, lipids, and ATP can be formed under plausible prebiotic conditions within a short timescale (roughly one week in experiments).
      2) Polymer formation (monomers → polymers).
    • The challenge is proximity: in a dilute ocean, monomers may not encounter each other to form polymers.
    • A proposed solution is that clays on early Earth’s crust, containing positively charged minerals like Fe$^{2+}$ and Zn$^{2+}$, could concentrate monomers and facilitate polymerization by bringing them into proximity.
      3) Formation of protobionts (lipid-based structures that can compartmentalize reactants).
    • Lipids form bilayers and can assemble into liposomes with an aqueous interior, providing a microenvironment and solving the proximity problem.
      4) Origin of hereditary material (RNA world hypothesis).
    • The first hereditary material is thought to be RNA rather than DNA because RNA can be formed abiotically more readily, can self-replicate under simple conditions, can store hereditary information, and can act as a catalyst.
    • There is accumulating evidence that RNA processing and synthesis could be possible under prebiotic conditions, with hints from viroids (tiny RNA replicators) that are smaller than typical viruses; Viroids can infect plants using as few as ~250 nucleotides (and sometimes as few as 50 nucleotides), illustrating that RNA-based replication and heredity is plausible in a minimal form.
  • What we still don’t know: the exact moment when life transitioned from non-life to living systems is not understood; this is a major area of ongoing research with many plausible interpretations.
  • The lecture emphasizes that the starting point for life (the universal common ancestor) remains a topic with substantial uncertainty, but what is clear is that life emerged around 3.8\times 10^{9} years ago and led to the vast diversity we observe today.
  • Viable synthesis and the RNA world are used to illustrate plausible prebiotic pathways toward life, while acknowledging that multiple branches of possibility exist.

Interlude: science, faith, and personal perspective

  • The lecturer notes balancing science with faith: as an evolutionary biologist who is also a practicing Catholic, science and faith can inform each other and coexist.
  • The instructor invites students to discuss how to reconcile scientific and theological perspectives.

Quick takeaways and connections to subsequent topics

  • Earth’s history is characterized by long periods (Precambrian) followed by shorter, punctuated events (Cambrian explosion, mass extinctions like the Permian and KT events).
  • Life began in a chemically favorable, reducing environment, with early organisms dominated by unicellular life (bacteria and archaea), followed by the emergence of multicellular life and complex ecosystems.
  • Major milestones include the appearance of single-celled life (~3.8\times 10^{9}yearsago),multicellularlife( years ago), multicellular life (~1\times 10^{9}yearsago),landplants( years ago), land plants (~4.5\times 10^{8}yearsago),landanimals( years ago), land animals (~4.2\times 10^{8}yearsago),thePermianextinction( years ago), the Permian extinction (~2.52\times 10^{8}yearsago),theKTextinction( years ago), the KT extinction (~6.5\times 10^{7}yearsago),andtheriseofmammalsandhumansthereafter( years ago), and the rise of mammals and humans thereafter (~2\times 10^{8}yearsagoandyears ago and2\times 10^{6}$$ years ago, respectively).
  • The timeline also underscores how geological processes like continental drift, climate change, and atmospheric composition have shaped biological evolution.
  • Finally, the origin of life is framed as a four-step process leading from simple chemistry to protobionts and RNA-based heredity, with ongoing research to understand the precise sequence and timing.

Key terms to review

  • Protoplanetary disk, Hadean Eon, Archean Eon, Proterozoic Eon, Precambrian Supereon
  • Stromatolites, cyanobacterial mats, Archaeocyathids
  • Cambrian explosion, trilobites, Archaeocyathids, eukaryotes
  • Permian extinction (Great Dying), KT extinction, KT boundary
  • Continental drift, plate tectonics, geological clock
  • Reducing atmosphere, UV energy, prebiotic chemistry
  • Miller–Urey experiments, prebiotic synthesis, polymerization, proximity problem
  • Protobionts, liposomes, RNA world, ribozymes
  • Viroids, nucleotide counts, RNA heredity