Emergence of Life
How did life emerge on Earth? How have life and Earth co-evolved through geological time? Is life
elsewhere in the universe? This course reevaluates these questions and the 4 billion-year history of
life on Earth through the lens of the most up-to-date concepts of the "Tree of Life." The course
fundamentally reconsiders what early life and evolution looked like before the root of the Tree of
Life, the overall succession of life that has inhabited our planet, and the search for life in the cosmos.
Course Goals and Objectives
The goals of this course are to have participants gain a basic understanding and familiarity with the
emergence of life on Earth, along with an appreciation for how these perspectives are shaping our
search for life throughout the universe.
By the end of the course, participants will be able to:
1. Summarize the fundamentals of molecular phylogeny and the essence of the Tree of Life.
2. Name the major extinctions and radiations punctuated by meteor impacts through
geological time.
3. Compare the fossil record throughout the Phanerozoic.
4. Examine controversial questions in evolutionary biology and astrobiology.
5. Apply scientific inquiry to new observations and ideas not encountered before.
6. Conduct a basic analysis of a scientific research paper.
Textbook
There is no required textbook for this course. Any other reading materials will be listed in the weekly
overview or activity pages.
Course Outline
The course consists of 8 weekly modules, each of which will be released to you shortly before the
module begins.
Week 1: Geological Time and the Nature of Science
The week begins with a discussion of the historical and philosophical approaches that have been
developed for (and are now actively applied to) the completion of scientific research. If done well,
scientific research is both reproducible and predictive in nature. The work of Professor Carl Woese is
evaluated as an example. The basic tools required for this type of scientific reasoning and the ability
to overcome the challenging concepts of scale and complexity are then presented. We'll see how
NASA utilized these approaches to send a message into space. Next, we'll explore the dynamic
formation of the Earth itself with respect to the environmental conditions present on the earliest
and most ancient version of planet Earth.
Week 2: The Tree of Life and Early Earth Environments
The advent of life on Earth came about as a result of a remarkable confluence of physical, chemical
and biological processes, all of which were intrinsically linked to rapidly changing early Earth
environments. Within this context, cutting-edge approaches in molecular phylogeny have revealed
new understandings of the series of events associated with the emergence of life, and the possible
distribution of Life within the cosmos. In this process, early assemblages of biochemical components
(called the progenote) then transitioned into individual living cells. These cells then evolved to form
the Tree of Life (Bacteria, Archaea and Eucarya). From this time forward, Earth environments directly
controlled life, but in turn life rose to directly control Earth environments. A key example was the
evolution of photosynthesis by the Bacteria to oxygenate the atmosphere 2.5 billion years before
present, which then triggered microbial mat growth on the ancient seafloor. The end of this
Precambrian time period was associated with extreme global climate change.
Week 3: Fossilization and Precambrian Life-Earth Interaction
The accurate interpretation of ancient fossilized life requires the extreme development of crime
scene investigative approaches. This version of ancient forensic science yields remarkably detailed
and complete reconstructions of the lifestyles of ancient organisms that have been deceased for
hundreds, thousands, millions, and even billions of years. These reconstructions, when conducted
within the context of ancient environmental conditions, provide valuable information regarding the
evolutionary success of organism morphology and lifestyle. Therefore, “survival of the fittest”
becomes “survival of those fit best.” The first great natural experiment in this respect, run by the
lineage of the Eucarya, was the Ediacaran fauna at the end of the Precambrian.
Week 4: Paleozoic Life After the Advent of Skeletons
A watershed event in the evolution of life on Earth was the development of external hard skeleton
components, called the Cambrian Explosion, at 542 million years before present. The initial
successes of the invertebrates were shortly followed by the appearance of vertebrates with internal
skeletons. Life then utilized these new-found evolutionary capabilities, beginning distinct cycles of
radiation, diversification, and extinction, which define the 3 great Eucarya faunas of the Phanerozoic.
A benchmark event in this process was the evolution of jaws and its ensuing influence on predation.
Week 5: Paleozoic Plants, Reptiles, and the Transition to Land
In the early Paleozoic, plants evolved to leave the water and invade the terrestrial landscape. The
shift meant an abundant food source was readily available on land, and was followed by the
transition of vertebrates into land-based ecosystems. Global increases in CO2 and associated
greenhouse warming in the Pennsylvanian further propelled the success of both prevascular and
vascular plants and the related terrestrial radiation of the 3 lineages of the reptiles (Anapsids,
Synapsids, and Diapsids). While Synapsid predators dominated the late Paleozoic, a massive meteor
impact and later environmental changes reset the ecological and evolutionary stage, opening the
door to the rise of the Diapsids.
Week 6: Mesozoic Reign of the Dinosaurs and the Development of Flight
More than 80% of life was extinguished during the Permian-Triassic meteoric impact event, opening
vast swaths of ecological opportunity for radiation and diversification of life during the Mesozoic.
Symbiosis, the mutually beneficial association of two living organisms, was widely utilized during this
time period. A fascinating lineage of Diapsid reptiles rose to replace the Synapsid predators of the
late Paleozoic, evolving multiple distinct types of reptiles. Flight evolved in the process, allowing
Pterosaurs to have a global reach in their annual migrations. Radiation and diversification continued,
with the emergence of the lineage of the Dinosaurs, Archaeopteryx and eventually the birds. The
end of the Mesozoic was ushered in by another catastrophic meteor impact, driving the dinosaurs to
extinction and opening the evolutionary stage for the Cenozoic.
Week 7: Cenozoic Mammals and Global Environmental Change
With the demise of the Dinosaurs, Mammals rapidly radiated and diversified during the Cenozoic.
The combination of abundant food sources and significant fluctuations in global climate fostered
extreme variations in morphology, body size, and interaction with the environment. One distinct
advantage that mammals had over previous groups was the increase in brain size within respect to
body size, evidenced by fossilized mammal skulls. Multiple theories on primate and hominoid
evolution are visited, focusing on recent evidence that sheds new light on intermediate transitional
forms in the lineage. This is contextualized with an overview of the extreme changes in global
climate that have taken place throughout the Cenozoic and into the modern day Earth.
Week 8: Astrobiology and the Search for Life in the Cosmos
All people ask the same fundamental questions: Where are we from? Where are we going? Is there
life elsewhere in the universe? The National Aeronautical and Space Administration (NASA) is
dedicated to searching for answers to these questions. The NASA Astrobiology Roadmap describes
our current state of knowledge regarding the origins, evolution, distribution, and future of life in
throughout the universe. These fundamental concepts of life and habitable environments,
established upon the modern synthesis of the Tree of Life, will direct us in recognizing biospheres
that might be quite different from our own. Previous, current, and future NASA missions continue to
fundamentally reset and enhance our ability to scientifically answer these vexing questions.