Life on Earth: A Journey Through Time
Formation of Earth and Early Life
Formation of Earth occurred approximately 4.6 billion years ago.
The lecture will cover from the Earth's formation to the recent evolution of humans.
The lecture will focus on the timeline in the textbook, discussing key events and their relationship to life and current biodiversity.
Topics to be covered include the origin of life, patterns of radiations and extinctions, evolutionary processes, and human evolution with recent findings.
Early Earth: the Archean era was red due to microbial life, global glaciation events occurred, and continents looked different (Pangaea).
The last ten thousand years (Holocene) have been significantly shaped by humans.
Timeline of Life
The timeline of life is split into four main eras: Hadean, Archaean, Proterozoic, and Phanerozoic.
These eras represent important periods with distinct events.
Hadean Era
Hadean (Greek for "hell"): Earth was a fiery, molten mass with a hostile environment for life.
Towards the end of this era, the planet cooled, leading to the emergence of the first prokaryotes.
The presence of prokaryotes is what made the planet reddish in color.
Origin of Life Theories
Meteorites may have delivered simple organic molecules needed for life.
Life requires simple organic molecules that can replicate, allowing natural selection and evolution to begin.
Evidence suggests meteorites containing amino acids like glycine collided with Earth.
The Miller-Urey experiment (1952) attempted to replicate early Earth conditions.
A chamber with a reducing atmosphere (gases present in early atmosphere) was subjected to electrical charges (lightning).
Gases were condensed and cooled, and a water source (ocean) was heated.
Over a week, the experiment produced small organic molecules.
Deep-sea hydrothermal vents: Compounds from the Earth interact, forming hydrogen compounds that mix with carbon dioxide, leading to self-forming protocells.
These protocells have a double membrane consisting of fatty acids and some RNA.
Prokaryotes
Earliest life forms are prokaryotes, possibly formed from protocells.
Basic Features of Prokaryotes
Single-celled organisms with no nucleus; instead, they have a nucleoid (simple ring of DNA).
Lack organelles and significant internal structure.
Do not have chloroplasts but have thylakoids for photosynthesis.
Cyanobacteria are important prokaryotes, capable of photosynthesis.
Cyanobacteria were observed under the microscope in the wet lab.
Cyanobacteria such as Ossalatoria has chlorophyll distributed throughout the cell without clear packaging (no chloroplasts).
Prokaryotic Life on Early Earth
Cyanobacteria bound with sediments, forming stromatolites (layered) or thrombolytes (cauliflower-shaped).
Dominant organisms of the Proterozoic era.
Significance of Cyanobacteria
Photosynthesis: A major biological innovation.
Photosynthesis produces oxygen, which is crucial for other life forms.
The basic equation for photosynthesis:
This process led to significant changes during the Proterozoic era.
Proterozoic Era
Proterozoic (early life) is marked by the rise of eukaryotes following the increase in atmospheric oxygen.
Characteristics of Eukaryotes
Eukaryotes are more recent than prokaryotes (appearing about 1.5 billion years ago compared to 3.5 billion years ago).
Can be single-celled or multicellular.
Possess a nucleus (containing DNA/RNA material) instead of a DNA ring.
Have complex organelles and internal structures like mitochondria, Golgi vesicles, and a plasma membrane.
Theory of Endosymbiosis
One organism lives inside another.
An ancestral eukaryote cell engulfs an aerobic bacterium, leading to a symbiotic relationship.
The aerobic bacterium becomes specialized as the mitochondria (powerhouse of the cell).
Engulfing a cyanobacterium (photosynthetic) leads to chloroplasts and the plant cell lineage.
Evolution of Multicellularity
Cells aggregate for defense against predation.
Aggregation allows functional specialization and information integration about the environment.
Slime molds demonstrate the evolution of multicellularity; they can move more efficiently towards food sources when multicellular.
Consequences of Multicellularity
Cooperation among cells influences cancer.
Unicellular organisms have a network of interacting genes.
Multicellular organisms have additional genes from different evolutionary origins.
Tumors may respond differently due to these different evolutionary origins, affecting gene regulation.
Understanding these evolutionary processes is important for understanding phenotypic and genotypic changes in cancers.
Phanerozoic Era
The Phanerozoic is the most recent 500 million years.
A popular way of thinking about it is if you think about the history of the Earth in terms of a twenty-four hour clock and the events that happened in terms of that.
The Phanerozoic is really very recent, it occurs about 09:00 at night, and the dinosaurs went extinct in November; human history occurs in just 0.2 seconds before midnight.
Cambrian Explosion
A rapid evolutionary period with a burst of new life forms around 540 million years ago.
Discovered in Canada, where soft sediments preserved animals well (Burgess Shale).
The precursors of many animal groups appeared during this explosion (arthropods, mollusks, echinoderms, chordates).
Animals evolved bilateral symmetry, paired eyes, and paired appendages.
Bilateral symmetry allows for increased body organization, sensory systems, and tissue organization.
Evolution of eyes was hugely important, improving sensory capabilities.
Possible drivers: increased nutrients, changes in Earth's atmosphere, and an explosion of oxygen.
Complete food webs with predator-prey interactions emerged.
Land Plants
The emergence of land plants followed. The first land plants were rudimentary, without roots.
Increased sedimentation from weathering and geological processes facilitated their expansion.
Land plants released phosphorus, which is important nutrient for other organisms and allowing the development of food chains.
Tetrapods
Tetrapods (four-limbed organisms) arose from sarcopterygians (ancient fish with fleshy, lobed fins).
Pelvic fins evolved into homologous structures, like the forelimbs and hind limbs.
Early amphibians led to later tetrapods; the earliest were amniotes (developing in a fluid-filled sac).
There were two main radiations into the synapsids and the sauropsids.
Synapsids formed the mammals (single opening in the skull), and sauropsids formed the reptiles and birds (two openings in the skull).
Marsupial Mammals and Flowering Plants
Later on, around 150 million years ago, there was the appearance of marsupial mammals and flowering plants with subsequent radiation.
However, this rate was slower compared to the Cambrian explosion.
Diversification rates of flowering plants are linked with environmental and geographical parameters; the mean diversification rate is shaped by climate globally.
Extinctions
Earth has been shaped by major extinction events.
There have been five major extinctions: the Permian and Triassic, KT (Cretaceous-Tertiary), Ordovician, Devonian and the Cretaceous.
Permian-Triassic Extinction
End of this major extinction was brought on by a change in atmospheric carbon.
Caused by increased carbon dioxide, particularly from volcanic sources.
Led to the loss of all trilobites, 90% of marine species, and 70% of terrestrial vertebrates.
KT Extinction
The KT extinction happened around 65 million years ago.
75-80% of species were lost including all non-avian dinosaurs and many invertebrates over 25 kilograms.
Major extinctions are associated with Earth's climate, including periods of glaciation and heating.
Biogeography and Continental Drift
Continental plate tectonics are important for biology and understanding animal distributions.
Earth is delineated into six zones: paleo Arctic, Ethiopian, Australian, Neo Arctic, and neo tropical regions along with the Arctic.
Birds and reptiles split up about 300 million years ago.
Following that, we have three different groups of mammals that have formed the prototheria (monotremes), the metatheria (marsupials) and the eutheria (placental mammals).
The breakup of Pangaea led to the current continental arrangement, influencing the evolution of mammals.
Marsupials radiated in North America, moved to Europe and Africa, but later became extinct there; they migrated through South America to Australia, where they persist due to the continent's isolation.
Human Evolution
Humans are a recent appearance (0.2 seconds before midnight on the 24-hour scale).
Humans are a member of the hominids.
Humans share around 99% of their genes with chimpanzees.
Hominins diversified rapidly within the last ten million years.
Australopithecus, including the famous fossil Lucy (Australopithecus afarensis), is a well-known hominin.
Homo evolved around two million years ago.
Species of Homo, including Homo sapiens, Homo neanderthalensis, and Homo floresiensis, coexisted.
Interbreeding occurred between different Homo species.
Non-African people have around 2-3% of their genes from Neanderthals.
Mixing between Neanderthals and modern-day humans happened around 40 to 50,000 years ago.