History of Life on Earth: Videos & Practice Problems

The history of life on Earth is shaped by plate tectonics and continental drift, leading to biogeography, where species evolve differently due to geographic separation. The fossil record, though biased, reveals ancient life, with radiocarbon dating helping to determine ages. Mass extinctions, like that of the dinosaurs, allow for adaptive radiations, where surviving species diversify. Key events include the Cambrian explosion and the emergence of eukaryotes through endosymbiosis, illustrating punctuated equilibrium in evolution, marked by rapid changes followed by stability.

Geologic Record

The history of life on Earth spans approximately 4.5 billion years, a timeline that can be visualized as a calendar year, where January 1st represents the formation of the planet and December 31st marks the present day. Life is believed to have originated around 4 billion years ago, but the earliest fossils, known as stromatolites, date back to about 3.5 billion years ago, roughly corresponding to March 25th on our calendar. These fossils are formed by prokaryotic organisms, which are simple, single-celled life forms.

A significant milestone in Earth's history is the Great Oxygenation Event, occurring between 2.7 and 2.4 billion years ago. This event marks the emergence of photosynthetic prokaryotes, which began producing oxygen as a byproduct of photosynthesis. By June 19th, 2.4 billion years ago, oxygen levels in the atmosphere increased dramatically, creating a challenging environment for anaerobic organisms that could not tolerate oxygen.

As we progress through the timeline, eukaryotes, which are more complex cells with nuclei and organelles, appeared around 1.8 billion years ago, equivalent to August 8th. The evolution of eukaryotes is attributed to endosymbiosis, a process where prokaryotes were engulfed by larger cells, eventually becoming mitochondria and chloroplasts. The first multicellular eukaryotes emerged about 1.3 billion years ago, around September 18th, represented by a form of red algae.

The Cambrian explosion, occurring approximately 535 to 525 million years ago (around November 19th), was a period of rapid diversification of life forms, particularly animals. This event saw the emergence of recognizable animal life, moving beyond simple organisms like sponges. Following this, the colonization of land by eukaryotes, including the first plants, animals, and fungi, took place about 500 million years ago, on November 21st.

Humans, however, are a relatively recent addition to this timeline, appearing around 200,000 years ago, which corresponds to December 31st at 11:40 PM. Recorded human history spans only the last 5,000 to 6,000 years, or the final 30 seconds of the year. This perspective highlights the vastness of Earth's history and the relatively late emergence of humans in the grand timeline of life.

History of Life on Earth Example 1

Earth's history is divided into four major eons: the Hadean, Archaean, Proterozoic, and Phanerozoic, spanning approximately 4.5 billion years. Understanding the timeline of significant events in the history of life helps to contextualize these eons. The Hadean eon, which lasted from the formation of Earth to about 4 billion years ago, was characterized by extreme conditions that were inhospitable to life. The first evidence of life, in the form of definitive fossils, emerged around 3.5 billion years ago, marking the beginning of the Archaean eon.

Following the Archaean, the Proterozoic eon saw the evolution of multicellular organisms, which appeared approximately 1.3 billion years ago. This transition from unicellular to multicellular life involved the development of eukaryotes, which are complex cells with a nucleus. The Cambrian explosion, a significant event that occurred around 535 to 525 million years ago, marked a rapid diversification of animal life and is considered the beginning of the Phanerozoic eon.

In the context of land colonization, eukaryotes began to inhabit terrestrial environments shortly after the Cambrian explosion, around 500 million years ago, also placing this event within the Phanerozoic eon. Thus, the timeline of these key events aligns as follows: the first definitive fossils in the Archaean, the emergence of multicellular organisms in the Proterozoic, and both the Cambrian explosion and the colonization of land by eukaryotes in the Phanerozoic.

Each option below lists several of the major events in the history of life on Earth. Which option presents the events in the correct order from the oldest to the most recent?

• A. Endosymbiosis leads to the appearance of eukaryotes → Eukaryotes colonize land → Cambrian explosion.
• B. Prokaryotes form stromatolites → Multicellular organisms appear → First eukaryotes.
• C. Land colonization → Cambrian explosion → Endosymbiosis leads to the appearance of eukaryotes.
• D. Prokaryotes form stromatolites → Cambrian explosion → Eukaryotes colonize land.

Answer: (D)

Here’s what students ask on this topic:

What is the theory of plate tectonics and how does it relate to the history of life on Earth?

The theory of plate tectonics posits that Earth's crust is divided into large, solid plates that float on the semi-fluid mantle beneath. These plates move over time, causing continental drift. This movement has significantly influenced the history of life on Earth by altering habitats and climates, leading to the geographic separation of species. This separation, in turn, has driven evolutionary processes, resulting in the diverse distribution of organisms we see today. For example, the breakup of the supercontinent Pangaea led to the isolation of species, which then evolved independently, contributing to biodiversity.

How does radiocarbon dating work and what is its significance in studying the history of life on Earth?

Radiocarbon dating measures the decay of carbon-14 (C-14) to estimate the age of organic materials. Living organisms constantly incorporate C-14, but when they die, the C-14 begins to decay into nitrogen-14 (N-14) at a known rate, called the half-life (about 5,730 years). By comparing the ratio of C-14 to carbon-12 (C-12) in a sample, scientists can estimate its age. This method is crucial for dating fossils and understanding the timeline of life on Earth, providing insights into when specific organisms lived and how they evolved over time.

What are mass extinctions and how have they impacted the evolution of life on Earth?

Mass extinctions are events where a significant percentage of Earth's species die out in a relatively short period. There have been five major mass extinctions, each reshaping life by eliminating dominant species and allowing others to thrive. For instance, the extinction of the dinosaurs, likely caused by an asteroid impact, paved the way for mammals to become the dominant terrestrial animals. These events often lead to adaptive radiations, where surviving species rapidly diversify to fill ecological niches left vacant by extinct species, driving evolutionary change.

What is the Cambrian Explosion and why is it significant in the history of life on Earth?

The Cambrian Explosion, occurring around 541 million years ago, was a period of rapid evolutionary diversification where most major animal phyla appeared. This event is significant because it marks the transition from simple, mostly unicellular life forms to complex, multicellular organisms. The Cambrian Explosion set the stage for the development of diverse life forms, including the ancestors of many modern species. It highlights the dynamic nature of evolution, where significant changes can occur in relatively short geological periods.

What is biogeography and how does it help us understand the distribution of species?

Biogeography is the study of the distribution of species and ecosystems across geographic space and through geological time. It helps us understand how species have spread and evolved in different regions, influenced by factors like continental drift, climate changes, and ecological interactions. By examining the geographic distribution of fossils and living organisms, biogeographers can infer historical connections between landmasses and the evolutionary pathways of species, providing insights into how life on Earth has diversified and adapted to various environments.