Chapter 25: The Origin and Evolutionary History of Life on Earth
What is the best current estimate of the age of the Earth, and what is the evidence for that estimate?
What are considered to be the four requirements for life to begin on Earth?
Describe the contributions of Oparin and Haldane and of Miller and Urey to models of the origin of life on Earth.
Briefly discuss at least three different models for how life began on Earth.
What are protobionts and microspheres, and what does their existence imply about how cellular life began?
Explain the RNA world hypothesis and how in vitro evolution tests it.
What are microfossils, and to what age to the oldest ones found on Earth date?
What are stromatolites?
What are banded iron formations, and why are they important?
When did oxygenation of Earth’s atmosphere occur, and what were the key consequences of it?
When do eukaryotic cells appear in the fossil record?
When was Precambrian time and what were the major events during that time period?
For each of the “big five” mass extinctions, give when they occurred, their likely causes if known, and key consequences of them.
Outline the relative history, dominant organisms, and key events of each of the periods of the Paleozoic, Mesozoic, and Cenozoic eras.
What is the sixth extinction?
Chapter 25: The Origin and Evolutionary History of Life on Earth
Chemical conditions of the early Earth that could have fostered the origin of life
the Earth is about 4.6 billion years old (time of the first likely solid surface)
supported by radioisotope dating of
oldest known Earth minerals (date to 4.4 billion years ago, or 4.4 bya)
oldest known rocks on Earth (4.1 bya)
oldest known meteorites (4.6 bya; for the age of the solar system)
some models go out to 6 billion years, it is hard make a rule for a definitive starting point of planet formation
Earth’s early atmosphere (when life first appears in the fossil record) most likely consisted of CO2, H2O, CO, H2, N2, and small amounts of NH3, H2S, and CH4 – note the lack of O2, which is a major constituent of today’s atmosphere
Four requirements for the current chemical evolution model were likely met in the early Earth
little or no free oxygen
abundant energy sources (volcanism, thunderstorms, and bombardment with particles and radiation from space were all likely present as energy sources; especially important is more UV radiation than today)
chemical building blocks of water, dissolved mineral ions, and atmospheric gases
time (there was plenty of time before the first traces of life from 3.8 bya])
attempts to mimic the early Earth’s atmosphere and chemical profile have led to production of organic molecules from simpler materials after energy is added
1920s – Oparin and Haldane independently proposed that organic molecules could form spontaneously from simpler raw materials when sufficient energy is supplied in a reducing (energy-rich, electron-adding) environment
1950s – Miller and Urey made a “reducing atmosphere” of H2O, H2, NH3, CH4 in a spark chamber; after sparking, they found that amino acids and other organic compounds had formed
later experiments with different “reducing atmospheres” that were thought to be better matches to the likely atmosphere of the early Earth produced all 20 amino acids used in proteins, various sugars and lipids, and components of DNA and RNA nucleotides
current models of the Earth’s early atmosphere are that in general the atmosphere was not reducing, but that there were likely many local environments that were reducing – especially near volcanic activity
organic polymers can form spontaneously from monomer building blocks on some sand, clay, or rock surfaces
there are several models for exactly where and how life as we know it on Earth began
prebiotic broth hypothesis – life began from an “organic soup” in the oceans
bubble hypothesis – a variation on the prebiotic broth, with “oily bubbles” from an organic soup interacting with land surfaces at shallow seas or seashores
iron-sulfur world hypothesis – life began from an “organic soup” interacting with mineral surfaces at hydrothermal vents in the ocean floor, with abundant iron and sulfur there impacting the early metabolism that developed
deep-hot biosphere hypothesis – life began in an “organic soup” deep within the Earth
exogenesis – Earth was seeded with life from an extraterrestrial source
eukaryotes first appear in the fossil record about 2 bya, long after prokaryotic cells
DNA sequencing provides evidence of common ancestry of life on Earth, with eukaryotes splitting from Archaea about 2 bya
recall the endosymbiotic theory – model for how at least some of the eukaryotic cell organelles came to exist
History of life on Earth (organized by divisions of geological time)
basis for the divisions
divisions of geological time are based major changes in types of organisms found in the fossil record (each division has its own characteristic set of commonly found fossils and unique fossil forms)
many of the transitions between the divisions are marked by major extinction events, where many organisms apparently died out over a short period of time because they disappear form the fossil record from that point on
although there are many major extinction events in the fossil record, by most measures five stand out above the rest; you need to know these “big five” mass extinction events
Precambrian time, from 4.6 bya up to 542 million years ago (mya); the fossil record is very spotty prior to 542 mya
we have already covered some of the major events of that time period (origin of life, oxygenation of the oceans and atmosphere)
Devonian period (416-359 mya) – jawed fishes, amphibians, insects, and vascular plants first appear; jawed fishes diversify and dominate the seas (Age of Fishes), and vascular plants diversify and dominant the land
Devonian period ended in a mass extinction event (2nd of the big five)
Carboniferous period (359-299 mya) – reptiles first appear; amphibians diversify and are the dominant terrestrial carnivores (Age of Amphibians); most of today’s major coal deposits are the remains of organisms that lived in this period
Permian period (299-251 mya) – by the end of this period, the continents have merged as the Pangaea supercontinent
the era ended (251 mya) with a mass extinction event (3rd of the big five)
the largest mass extinction on record
more than 90% of the marine species and 70% of land vertebrates that are in the fossil record at the end of the Permian never appear in the fossil record again
the mass extinction event apparently took place in a time span of only a few hundred thousand years, which is fast in the geological time scale
Mesozoic era (251-65 mya)
diversification and dominance by reptiles – the whole era is often called the Age of Reptiles (sometime called the Age of Dinosaurs, many non-dinosaur reptiles were prominent)
Triassic period (251-200 mya)
dinosaurs and mammals first appear
gynmnosperms are the dominant land plants
ended with a mass extinction event (4th of the big five) that paved the way for the dinosaurs to rise to prominence
Jurassic period (200-146 mya)
dinosaurs dominate the land (and other large reptiles dominate the seas and the skies)
birds evolve from a dinosaur lineage
Cretaceous period (146-65 mya)
flowering plants evolved around the early Cretaceous and diversified quickly
many animals (especially insects) appear to have coevolved with flowering plants (different species affecting each other’s evolution)
the era ended (65 mya) with a mass extinction event (5th of the big five)
dinosaurs essentially all died out (unless you count birds as dinosaurs, which some scientists do)
evidence points to the impact of a large extraterrestrial body as a likely cause of the extinction event
a major impact almost certainly occurred at this time; iridium layers worldwide and deposits from tsunamis around the Gulf of Mexico coast of the time provide clear evidence of this
a large crater site (Chicxulub crater) in the Yucatán Peninsula of Mexico is likely the result of this impact
the extent to which such an impact could affect the biosphere is still debated, but is accepted by more and more scientists as at least a contributing factor to this massive extinction event
Cenozoic era (65 mya – present)
usually called the Age of Mammals, but birds, insects, and flowering plants have also undergone massive diversification and have all achieved some measure of “dominance” in the biosphere during this era
two periods, Paleogene (65-23 mya) and the Neogene (23 mya – present), although an older division into Tertiary (65-~2 mya) and Quaternary (~2 mya – present) is still often referred to
the Neogene has been marked by many ice ages, the rise of humans, and mass extinctions; most of these mass extinctions may have been caused by the ice age climate, humans, or both; the current mass extinction event (we are in one now, the sixth extinction) is mostly caused by humans