Origin of Life: Protobionts, RNA, and Early Evolution

Introduction to the History of Life on Earth

• The study examines major events in Earth's history, from the origin of life to present times.
• Life's history is divided into eons, eras, and periods.
• The fossil record provides evidence for life's history.

Overview: Lost Worlds

• Past organisms differed significantly from those currently alive.
• The fossil record demonstrates macroevolutionary changes over vast time scales.
  • These changes include the emergence of terrestrial vertebrates, the origin of photosynthesis, and long-term impacts of mass extinctions.

Macroevolution: Large Scale Changes Over Time

• Macroevolution encompasses large-scale evolutionary changes.

Concept 25.1: Conditions on Early Earth Made the Origin of Life Possible

• Chemical and physical processes on early Earth likely created simple cells through stages.
  • Abiotic synthesis of small organic molecules
  • Joining of these molecules into macromolecules
  • Packaging of molecules into protobionts
  • Origin of self-replicating molecules

Synthesis of Organic Compounds on Early Earth

• Earth formed 4.6 billion years ago.
• Early Earth's atmosphere contained water vapor and various chemicals, released from volcanic eruptions.
• Oparin and Haldane hypothesized that early Earth's atmosphere was a reducing environment.
• Miller and Urey conducted experiments showing the potential for abiotic synthesis of organic molecules in a reducing atmosphere.
• However current evidence isn't entirely convincing. The first organic compounds may have formed near volcanic vents.

Abiotic Synthesis of Macromolecules

• Small organic molecules can polymerize on hot surfaces like clay or rock.
• Protobionts are aggregates of abiotically produced molecules, surrounded by a membrane or membrane-like structure. These exhibit simple reproduction and metabolism.
• Experiments demonstrate protobionts can spontaneously form from organic compounds. Liposomes form from lipids in water is one example.

Self-Replicating RNA and the Dawn of Natural Selection

• RNA, not DNA, was likely the first genetic material.
• RNA molecules called ribozymes can catalyze many reactions, that may include their own copying, or of other RNA pieces.

Sedimentary Rocks and Fossils

• Sedimentary rock layers reveal the relative ages of fossils.
• Radiometric dating determines the absolute ages of fossils.
  • A "parent" isotope decays to a "daughter" isotope at a constant rate. Each isotope has a known half-life, used to determine the time elapsed since the decay began,.
• Radiocarbon dating is used for up to 75,000-year-old fossils.

The Origin of New Groups of Organisms

• Mammals belong to the tetrapod group of animals.
• Mammalian characteristics evolved gradually from ancestral synapsids.

Evolution of Mammals

• Synapsids and gradual change

Concept 25.3: Key Events in Life's History

• Life's history encompasses single-celled and multicellular organisms, and their colonization of land.
• The geologic record is divided into the Archaean, Proterozoic, and Phanerozoic eons.
• The Phanerozoic eon includes multicellular eukaryotic life divided into the Paleozoic, Mesozoic, and Cenozoic eras.
• Major boundaries/divisions correspond to extinction events.

Geologic Record

• Table of time, including the duration, periods, and major events in life's history.

The First Single-Called Organisms: Prokaryotes

• Stromatolites, layered rock structures, are the oldest known fossils.
• They are composed of bacteria and sediment dating back 3.5 billion years.
• Prokaryotes were the sole inhabitants of Earth from 3.5 billion to 2.1 billion years ago.

Photosynthesis and the Oxygen Revolution

• Most atmospheric oxygen comes from biological processes.
• Oxygenic photosynthesis produced oxygen that reacted with dissolved iron, creating banded iron formations.
• The likely source is similar to modern cyanobacteria.

By about 2.7 billion years ago

• Oxygen began accumulating in the atmosphere and rusting iron-rich terrestrial rocks.
• This oxygen revolution from 2.7 to 2.2 billion years ago posed a challenge for life and allowed organisms to exploit new ecosystems.

The First Eukaryotes

• The oldest fossils of eukaryotic cells date back 2.1 billion years.
• Endosymbiosis proposes that mitochondria and plastids were formerly small prokaryotes living inside larger host cells.

Serial Endosymbiosis

• Mitochondria likely evolved before plastids due to endosymbiosis, a process where interdependent host and symbiont become a single organism.

Invagination of Plasma Membrane

• Invagination of plasma membrane led to the development of organelles like endoplasmic reticulum and a nucleus

Changes in Spatial Pattern- Hox genes

• Alterations in homeotic genes (like Hox genes) can significantly affect body form and spatial structures like where wings, limbs and even flowers develop on organisms.
• If altered in the wrong location an organism could develop limbs where a head is required etc. Hox genes provide positional information during development. Hox gene duplications in the vertebrate lineage have been important in the evolution of new characteristics.

The Origin of Multicellularity

• The evolution of eukaryotic cells expanded the range of unicellular forms.
• Multicellularity led to the diversification of algae, plants, fungi, and animals.
• Comparisons of multicellular eukaryotic DNA sequences date the common ancestor to 1.5 billion years ago.

The "Snowball Earth" Hypothesis, The Cambrian Explosion, and Predator-Pray Interactions

• Periods of extreme glaciation confined life to equatorial regions or deep-sea vents during the late Precambrian.
• The Cambrian explosion was marked by a rapid increase of diversity and animal forms during the Cambrian period (~535-525 million years ago). Fossils in China provide evidence of modern animal phyla tens of millions of years earlier.
• The Cambrian explosion also provides the first reliable evidence of predator-prey interactions

The Colonization of Land

• Fungi, plants, and animals began to colonize land around 500 million years ago.
• Plants and fungi likely colonized land together by 420 million years ago.
• Arthropods and tetrapods are the most diverse and widespread land animals.
• Tetrapods evolved from lobe-finned fishes.

Concept 25.4: The Rise and Fall of Dominant Groups

• Land masses have formed supercontinents multiple times (1.1 billion, 600 million, 250 million years ago).
• Continents move over the underlying mantle, through continental drift.

Consequences of Continental Drift

• Pangaea's formation (approx 250 million years ago) had a profound impact on Earth's climate and ecosystems.

History of Continental Drift

• A visual time line showing the major continental arrangements throughout history.

The Breakup of Pangaea and allopatric speciation

• The break-up of Pangaea likely led to allopatric speciation.
• The distribution of fossils reflects continental drift patterns.
• Fossil similarities in South America and Africa indicate these continents were once joined.

Mass Extinctions

• Mass extinctions have occurred several times throughout Earth's history.
• The mass extinctions that ended the Permian and Cretaceous periods involved the loss of many species.
• The causes of mass extinctions may include volcanism, leading to global warming and lower oxygen levels, or possibly large meteorite impacts.

Massive Meteorite Impact Evidence

• Iridium in sedimentary rocks suggests a large meteorite impact about 65 million years ago.
• The Chicxulub crater in Mexico provides strong evidence for this impact event.

Is a Sixth Mass Extinction Under Way?

• The current extinction rate is estimated to be 100 to 1000 times higher than the typical background rate.
• Data suggest that a sixth human-caused mass extinction is possible, unless humans take dramatic action.
• Mass extinction can alter ecological communities, and niches. Recovery typically takes millions of years.

Adaptive Radiations

• Adaptive radiation is the evolution of diverse, new species from a shared ancestor in response to new ecological opportunities.

Adaptive Radiations: New Environmental Opportunities

• Mammals thrived after the extinction of dinosaurs, which created new niches.
• Other significant radiations include photosynthetic prokaryotes, large predators during the Cambrian, land plants, insects, and tetrapods.

World Wide Adaptive Radiations

• The adaptive radiation of mammals from the earliest mammalian ancestors, along with a time line.

Regional Adaptive Radiations

• Adaptive radiations can also occur in regions with less competition, such as the Hawaiian Islands.

Hawaiian Islands - Regional Adaptive Radiations

• The many species of Dubautia on the Hawaiian Islands.

Major changes in body form

• These changes result from changes in developmental genes, like the rate or timing of these genes or alterations in the placement and organization of body parts.

Heterochrony

• Heterochrony is an evolutionary change in the rate or timing of developmental events. Significant body shape changes can be due to this.

Paedomorphosis

• In paedomorphosis, the rate of reproductive development accelerates compared with somatic development. The adult species may retain juvenile body features from its ancestors.

Changes in Spatial Pattern - Hox Genes

• Hox genes provide positional information, determining the placement of body parts during development.
• Alterations in Hox gene expression can lead to substantial evolutionary changes in body form. The evolution of vertebrates is linked to alterations in Hox genes. Vertebrate lineages have had, at least, two duplications of Hox genes, possibly contributing to new characteristics. Vertebrates like fish, and mammals have undergone numerous morphological innovations due to alterations in Hox genes. Changes in developmental genes can result in new morphologies (physical shapes/structures).

Evolution is not goal oriented

• Evolution is essentially tinkering, modifying existing forms slowly to create new forms. New forms arise from slight modifications in existing ones over many stages. Complex structures like eyes and adaptations evolve incrementally, built on earlier structures.

Evolution

• The visual evolution of eyes through slight modifications

Horse Evolution

• Timeline and evolutionary relationships, showing how horses have appeared through time and morphological changes in their evolution.

Summary of Evolutionary Trends

• The evolution of life is not characterized by a predetermined trajectory but rather by patterns of diversification and speciation, shaped by a myriad of factors.

Key Terms and Concepts

-Define, and identify essential terms and concepts associated with the history of life on Earth.

Explore the origins of life, from the reducing early atmosphere to the significance of protobionts and RNA. Understand the evolution of genetic material and the diversification of animal life. Study the interconnectedness of early organisms in the colonization of land.