Bio 1300, Ch 26 History of Life on Earth
Chapter Overview
Title: History of Life on Earth and Human Evolution
Author: Dr. Troxell-Smith
Important Dates in Earth's History
Big Bang: Occurred approximately 13.7 billion years ago, marking the inception of the universe and the formation of matter.
Formation of the Solar System: Took place around 4.6 billion years ago, leading to the creation of the Sun and planets, including Earth.
Age of Earth: Estimated at 4.55 billion years, with geological processes shaping its surface over vast timescales.
Cooling and Solidification of Earth: By 4 billion years ago, Earth's surface had cooled sufficiently to solidify, and primordial oceans began to form, creating conditions suitable for life.
Emergence of Life: Life is believed to have first appeared between 4 and 3.5 billion years ago, with the earliest evidence pointing to simple, single-celled organisms.
Major Phases of Life
Pikaia gracilens: An early chordate, crucial for understanding the evolutionary lineage of vertebrates, representing one of the earliest known organisms with a notochord.
Transition of Life Forms: Life evolved from prokaryotic organisms to multicellular ones, illustrating a transition over billions of years.
First Vertebrates and Insects: Included early fish and primitive insects, which emerged around 400 million years ago, adapting to diverse environments.
Evolution of Reptiles, Dinosaurs, and Mammals: Following the Carboniferous period, reptiles became dominant, leading to the age of dinosaurs. Mammals began to diversify significantly following the mass extinction event that marked the end of the Mesozoic era.
Development of Flowering Plants: The rise of angiosperms around 140 million years ago drastically changed ecosystems, illustrating co-evolution with pollinators such as bees, which contributed to their rapid diversification.
Fossils and Paleontology
Fossils
Definition: Fossils are the preserved remains or traces of ancient life, offering insight into past biological processes and environments.
Paleontology: The study of fossils is predominantly conducted by paleontologists, focusing on sedimentary rock layers where fossils are typically found.
Conditions for Fossilization: Key factors include rapid burial to protect organisms from decay and the presence of hard body parts. Generally, older organisms are found deeper within geological strata, allowing for insights into evolutionary history.
Radioisotope Dating
Technique Overview: This method utilizes the decay of radioactive isotopes to determine the age of fossils, providing a chronological context for evolutionary timelines.
Half-life Concept: The half-life is the time required for half of a radioactive isotope to decay, offering measurable intervals for dating artifacts and fossils.
Insights and Limitations: While radioisotope dating provides valuable information, it tends to underestimate the actual timeline of species existence since it is based on death rather than living population dynamics.
Incomplete Fossil Record
Factors Affecting Fossilization:
Anatomy: Organisms with hard parts (bone, shells) are more likely to be fossilized compared to soft-bodied entities.
Size: Larger organisms tend to fossilize better than smaller, less detectable ones.
Environmental Context: Aquatic species often have better prospects for sedimentary preservation compared to terrestrial organisms, due to sedimentation processes.
Time: More recent species have a greater chance of being preserved as fossil evidence due to proximity to current geological activity.
Geological Processes: Various geological conditions can promote or hinder preservation, impacting the available fossil record.
Geological Time Scale
Major Time Divisions
Eons: Divided into major eons including Hadean, Archaean, Proterozoic, and Phanerozoic, the latter representing the time of visible life (around 541 million years ago).
Division into Periods and Events: Notably, the Cambrian Explosion around 543 million years ago marked a rapid diversification of life forms, laying the groundwork for modern ecosystems.
Environmental Changes Affecting Evolution
Major evolutionary shifts have been driven by changes in climate, tectonic movements, and cataclysmic events like mass extinctions.
Recognized Five Mass Extinction Events: These include events in the Ordovician, Devonian, Permian, Triassic, and Cretaceous periods, each resulting in significant losses in biodiversity.
Current Extinction Rates: Current extinction rates, largely fueled by human activity, may lead to a potential sixth extinction, raising concerns among scientists regarding biodiversity loss.
Evolution of Human Species
Origins of Hominids
First hominids appeared around 7 million years ago, with modern Homo sapiens emerging roughly 200,000 years ago, characterized by advanced cognitive abilities and culture.
Our evolutionary ties with nonhuman relatives highlight distinct characteristics such as bipedalism and increased brain size, fostering adaptability and social complexity.
Primate Evolution
Defining Features of Primates: Includes grasping hands adapted for manipulation, larger brains for enhanced cognitive processes, and intricate social interactions.
Major Taxonomic Groups: Primates are divided into two major groups, Strepsirrhini (lemurs, lorises) and Haplorrhini (monkeys, apes), differentiated based on nose structure and behavioral traits.
Key Characteristics of Hominids
Tribes within Hominidae: The tribe Hominini comprises modern humans and our direct ancestors.
Australopithecus afarensis: Notable species exemplified by Lucy, indicating early bipedalism and adaptation to terrestrial life.
Homo Species: Marked by increased cranial capacity, tool usage, and social cooperation, showcasing a trajectory of cognitive and cultural evolution.
Genetic Variation in Modern Humans
Ongoing Evolution: Humans continue to evolve, with traits such as disease resistance and lactose tolerance reflecting adaptations to diverse environments and dietary opportunities.
Genetic Variation: Although genetic variation among modern humans is relatively minimal, it primarily arises from single nucleotide polymorphisms (SNPs), revealing insights into our evolutionary past.
Conclusion
The evolutionary history of life underscores the interconnectedness of all organisms through time, demonstrating that modern humans are a product of millions of years of evolution, having diverged from ancestral primates and continuously adapted to the shifting conditions of our planet.
Lecture Questions
26.1
How old is the Earth?
4.55 Billion years old
What was the oldest fossil ever discovered? Why do we think that this is how life began?
The oldest fossils are stromatolites, which are layered structures created by the activity of microorganisms, and they date back around 3.5 billion years. These fossils provide evidence that life began as simple, single-celled organisms in ancient environments.
Fossils are discovered most frequently in which types of rocks?
Fossils are discovered most frequently in sedimentary rocks. These rocks form in layers from sediment accumulation, providing the right conditions for the preservation of organisms.
Where do scientists have to look to find the oldest fossils? What about newer fossils?
Scientists typically look for the oldest fossils in ancient sedimentary rock formations, often found in regions that have remained geologically stable. Newer fossils, on the other hand, can be found in rock layers closer to the Earth's surface.
What factors contribute to an incomplete fossil record?
Several factors lead to an incomplete fossil record:
Anatomy: Organisms with hard body parts (bones, shells) are more likely to be preserved compared to soft-bodied organisms.
Size: Larger organisms are typically more detectable and likely to fossilize than smaller ones.
Environmental Context: Aquatic species often have better prospects for fossilization due to sedimentation processes than terrestrial organisms.
Time: More recently existed species have a higher chance of being preserved due to proximity to active geological processes.
Geological Processes: Various geological conditions can either promote or hinder preservation, affecting fossil availability.
26.2
•What are the 4 eons?
Hadean
Archaean
Proterozoic
Phanerozoic
•What primary factors impact when one time frame ends and another begins?
Major geological, climatic, and biological events, such as significant shifts in biodiversity, mass extinction events, or major changes in Earth’s atmosphere and geology, can determine when one time frame ends and another begins.
•How many mass extinctions has the Earth gone through?
The earth has gone through five recognized mass extinction events.
•During what eon was the first known fossils discovered? Was it prokaryotic or eukaryotic and what is the difference between the two?
The first known fossils were discovered during the Archaean eon and were prokaryotic. Prokaryotic organisms are simple, single-celled organisms without a nucleus, whereas eukaryotic organisms have a more complex cell structure with a nucleus and organelles.
•During which eon was multicellularity discovered? Why was this an important step in evolutionary history?
Multicellularity first appeared in the Proterozoic eon. This was a significant evolutionary step as it allowed for greater complexity in organism structure and function, leading to the development of diverse life forms.
•What is the Proterozoic eon known for?
The Proterozoic eon is known for the rise of multicellular organisms and the buildup of atmospheric oxygen (The Great Oxidation Event).
•What is the Cambrian Explosion? When did it occur? Why was this an important step in evolutionary history?
The Cambrian Explosion occurred around 543 million years ago and marks a rapid diversification of life forms. This event laid the groundwork for modern ecosystems, featuring an increase in the complexity and variety of organisms, particularly in the marine environment.
26.3
•When is it believed that the first hominids appeared?
It is believed that the first hominids appeared around 7 million years ago.
•What successful features of humans do we owe to our primate cousins?
Humans owe several successful features to our primate cousins, such as advanced cognitive abilities, social behaviors, and adaptations for bipedalism.
•What are the 2 major groups of primates? How do they differ?
The two major groups of primates are:
Strepsirrhini (e.g., lemurs, lorises)
Haplorrhini (e.g., monkeys, apes)They differ mainly in their nose structure and behavioral traits.
Strepsirrhini have a wet nose and rely more on their sense of smell, while Haplorrhini possess a dry nose and have enhanced vision, often exhibiting more complex social behaviors.
•What species are included in Anthrpoidea?
The group Anthropoidea includes monkeys, apes, and humans.
•How do hominoids differ from monkeys?
Hominoids (apes and humans) differ from monkeys in that they do not have tails and tend to have larger brains relative to body size.
•What 2 families belong to the group Hominoids?
The two families that belong to the group Hominoids are:
Hylobatidae (gibbons, or lesser apes)
Hominidae (great apes, including humans).
•What species belong to subfamily Homininae?
The subfamily Homininae includes modern humans (Homo sapiens) and our direct ancestors, including species like Australopithecus.
•What key characteristic set hominins apart from other apes?
A key characteristic that sets hominins apart from other apes is bipedalism, which allows for walking on two legs.
•When did our species (Homo sapiens) arise?
Homo sapiens arose roughly 200,000 years ago.
•What hypotheses are used to describe how humans dispersed from Africa? Which has more scientific support and why?
Several hypotheses describe how humans dispersed from Africa, including the "Out of Africa" model and the "Multiregional" model. The "Out of Africa" model has more scientific support, as genetic evidence suggests that modern humans migrated from Africa to other parts of the world and replaced local populations.
•What are some examples of evolution within humans?
Examples of evolution within humans include traits such as disease resistance and lactose tolerance, which reflect adaptations to diverse environments and dietary opportunities.
•While we see a lot of phenotypic variation within humans, is that reflective of presence of genetic variation? Why?
While there is a lot of phenotypic variation within humans, it is not entirely reflective of genetic variation. Most genetic variation among humans is relatively minimal and primarily arises from single nucleotide polymorphisms (SNPs).
•What is cultural evolution? Why is it so important to humans?
Cultural evolution refers to the development of ideas, behaviors, and customs that are passed through learning rather than genetics. It is important to humans because it has allowed for the transmission of knowledge, adaptation to changing environments, and the development of complex societies.