Rucete ✏ Campbell Biology In a Nutshell
Unit 4 MECHANISMS OF EVOLUTION — Concept 25.3 Key Events in Life’s History Include the Origins of Unicellular and Multicellular Organisms and the Colonization of Land
Life on Earth evolved through several major events, from the appearance of simple unicellular organisms to complex multicellular life forms and eventual colonization of terrestrial habitats.
The First Single-Celled Organisms
Earth's first life forms were single-celled prokaryotes (~3.5 billion years ago).
Fossil evidence comes from stromatolites, layered rock structures formed by prokaryotes binding sediments.
Prokaryotes dominated Earth for about 1.5 billion years.
Photosynthesis and the Oxygen Revolution
Photosynthesis by cyanobacteria (~2.7 billion years ago) significantly increased atmospheric oxygen (O₂).
The rise of oxygen triggered the "oxygen revolution," dramatically changing Earth's atmosphere and leading to mass extinction of anaerobic organisms.
Aerobic respiration evolved in surviving organisms, efficiently using oxygen to extract energy.
The First Eukaryotes
Eukaryotic cells first appeared around 1.8 billion years ago.
Eukaryotes evolved from prokaryotes via endosymbiosis, where mitochondria and plastids originated from small prokaryotes engulfed by larger host cells.
Endosymbiotic evidence includes similarities between mitochondria/plastids and bacteria in membrane structure, replication, DNA, and ribosomes.
The Origin of Multicellularity
Multicellular eukaryotes appeared about 1.2–1.3 billion years ago.
The earliest known multicellular fossils include simple red algae (~1.2 billion years old).
Diverse multicellular forms (algae, animals) appeared significantly later, during the Ediacaran period (635–541 million years ago).
The Cambrian Explosion
The Cambrian explosion (~535–525 million years ago) was a sudden burst of animal diversity.
Before the Cambrian, animals were mostly soft-bodied grazers or filter feeders.
Afterward, predators and complex defensive adaptations rapidly evolved.
Colonization of Land
Land colonization (~500 million years ago) by plants, fungi, and animals represented a major evolutionary milestone.
Early terrestrial adaptations included vascular tissues, waterproof coatings, and mutualistic fungal relationships (mycorrhizae).
Arthropods were among the earliest land animals (~450 million years ago), followed by tetrapods (~365 million years ago), leading eventually to modern vertebrates, including humans (~195,000 years ago).
In a Nutshell
Life evolved through key events—from simple prokaryotic beginnings, through atmospheric oxygenation and the rise of complex multicellular eukaryotes, culminating in the explosive diversification during the Cambrian period and eventual colonization of terrestrial habitats.