The history of Earth stretches back approximately 4.54 billion years, but the familiar world of continents, oceans, plants, animals, and complex ecosystems took an enormous amount of time to develop. Long before dinosaurs, flowering plants, or even the first recognizable animals appeared, Earth experienced profound geological, chemical, and biological transformations.
The earliest and longest chapter of this history is known as the Precambrian. It began with the formation of Earth and continued until approximately 538.8 million years ago, when the Cambrian Period began. During this immense interval, the planet developed its earliest crust, oceans, atmosphere, and continental foundations. Microscopic organisms emerged, photosynthesis transformed environmental conditions, and increasingly complex forms of life eventually appeared.
The Precambrian encompasses three major geological eons: the Hadean, Archean, and Proterozoic. Each represents a distinct stage in Earth's development, from the formation of a young planetary body to the emergence of complex cellular and multicellular organisms.
Although the Precambrian accounts for most of Earth's geological history, its fossil record is considerably less complete than that of later eras. Ancient rocks have been altered, destroyed, buried, or recycled through geological processes, making some events difficult to reconstruct.
Nevertheless, evidence preserved in ancient minerals, sedimentary formations, chemical signatures, and microscopic fossils allows scientists to investigate how Earth became a habitable planet and how the foundations of modern life developed.
What Was the Precambrian?
The Precambrian is an informal geological term describing the enormous span of Earth's history before the beginning of the Cambrian Period. It includes the planet's formation, the development of its earliest geological systems, the emergence of life, and the evolution of increasingly complex organisms.
Unlike the Jurassic or Ordovician, the Precambrian is not a single geological period. It encompasses three major eons: the Hadean, Archean, and Proterozoic. Together, these intervals cover nearly nine-tenths of Earth's history.
During much of this time, life was dominated by microorganisms, including bacteria and archaea. These organisms occupied aquatic environments and developed metabolic processes that gradually influenced the chemistry of oceans and the atmosphere.
Several major developments distinguish the Precambrian from later geological history:
- Earth formed and developed its internal structure.
- The earliest crust, oceans, and continental foundations emerged.
- Microbial life appeared and diversified.
- Photosynthetic organisms began releasing oxygen.
- Atmospheric oxygen increased significantly.
- Eukaryotic cells evolved, followed by increasingly complex multicellular organisms.
- Large, soft-bodied organisms became established during the Ediacaran Period.
The Precambrian is particularly important because it established the physical and biological conditions required for later evolutionary developments. Without the formation of stable environments, the development of oceans, and the long-term transformation of atmospheric chemistry, the subsequent history of complex life would have been very different.
When Did the Precambrian Begin and End?
The Precambrian began approximately 4.567 billion years ago, during the formation of the early Solar System and Earth. It ended approximately 538.8 million years ago, at the beginning of the Cambrian Period and the Phanerozoic Eon.
According to the International Commission on Stratigraphy, the Precambrian encompasses three major eons.
| Geological division | Approximate time range |
|---|---|
| Hadean Eon | 4.567–4.031 billion years ago |
| Archean Eon | 4.031–2.5 billion years ago |
| Proterozoic Eon | 2.5 billion–538.8 million years ago |
These divisions represent different stages in Earth's geological and biological development.
The Hadean Eon covers the earliest history of the planet, including its formation, intense internal heating, and the development of its initial geological structure.
The Archean Eon witnessed the development of ancient crust and oceans, alongside the earliest widely accepted evidence of life.
The Proterozoic Eon brought substantial changes in atmospheric chemistry, cellular evolution, continental development, and biological complexity.
The final part of the Proterozoic includes the Ediacaran Period, which lasted from approximately 635 to 538.8 million years ago. It is especially notable for the appearance of large, complex organisms preserved in the fossil record.
The boundary between the Precambrian and Cambrian marks a major transition in the geological record. Cambrian rocks contain a much more diverse and abundant record of recognizable animal fossils, although animal evolution began earlier.
Because the Precambrian covers such an enormous period, scientists divide it into smaller geological intervals to describe important changes more precisely.
The Hadean Eon: Earth's Formation and Earliest Development
The Hadean Eon represents the earliest stage of Earth's history. It began approximately 4.567 billion years ago and ended around 4.031 billion years ago. During this interval, the young planet underwent major physical and chemical changes that established the foundations of its internal structure.
Formation of the young planet
Earth formed within the early Solar System from dust, rock fragments, and other material surrounding the young Sun. Through gravitational attraction and repeated collisions, these materials gradually accumulated into a larger planetary body.
As Earth grew, collisions and the release of gravitational energy generated enormous amounts of heat. Radioactive decay and the separation of dense materials within the planet also contributed to its internal temperature.
Much of the early Earth was likely extensively molten, although the precise extent and duration of any global magma ocean remain subjects of scientific investigation.
Development of Earth's internal structure
As the young planet heated and partially melted, materials separated according to their density and chemical properties.
Dense iron-rich materials moved toward the center, contributing to the formation of Earth's metallic core. Lighter silicate materials remained in the outer regions and developed into the mantle and early crust.
This process, known as planetary differentiation, established the basic internal structure that continues to influence Earth's geological activity.
The Moon-forming impact
One leading explanation for the Moon's origin is the giant impact hypothesis. According to this model, a large planetary body, commonly called Theia, collided with the early Earth.
Material ejected during the collision entered orbit around Earth and eventually accumulated to form the Moon.
The giant impact hypothesis is supported by several lines of evidence, including similarities between the chemical compositions of lunar and terrestrial materials. However, the exact details of the collision and subsequent formation process continue to be investigated.
Early crust, atmosphere, and water
Following the earliest stages of intense heating, portions of Earth's outer surface gradually cooled and solidified. Volcanic activity released gases that contributed to the development of an early atmosphere.
Water vapor released through volcanic processes, together with other possible sources of water, contributed to the formation of Earth's early water reservoirs as the surface cooled.
Ancient zircon minerals provide important evidence that liquid water may have existed on Earth's surface more than four billion years ago. These minerals preserve chemical signatures that help scientists reconstruct environmental conditions during a time when very few original rocks survive.
The Hadean was therefore not simply a period of destruction. It was the stage during which Earth developed the basic planetary structure and environmental conditions that eventually allowed life to emerge.
The Archean Eon: Early Continents, Oceans, and Earth's First Life
The Archean Eon extended from approximately 4.031 billion to 2.5 billion years ago. During this interval, Earth's surface became increasingly organized into ancient continental regions, while oceans supported some of the earliest known biological communities.
Development of early continental crust
The Archean witnessed the formation and growth of ancient continental crust. Some of the oldest surviving continental materials are preserved within stable geological regions called cratons.
Cratons are ancient portions of continental lithosphere that have remained relatively stable over immense geological periods. They provide valuable evidence of early crust formation and tectonic development.
Examples of important Archean geological records occur in Western Australia, southern Africa, and parts of Canada and Greenland.
Although modern-style plate tectonics may not have operated in exactly the same way throughout the Archean, geological evidence indicates that crustal movement, deformation, volcanic activity, and recycling of surface materials were already occurring.
Early oceans and marine environments
Liquid water became an increasingly important component of Earth's surface environment. Ancient oceans provided habitats in which chemical reactions and biological processes could occur.
The early atmosphere contained little free oxygen compared with today's atmosphere. As a result, the chemistry of Archean oceans differed substantially from modern seawater.
Dissolved iron was more abundant in many ancient marine environments, and microorganisms adapted to conditions that would be unfamiliar to most modern complex organisms.
Evidence of early life
Some of the oldest widely accepted evidence for life comes from ancient rocks dating to approximately 3.5 billion years ago.
Scientists investigate this evidence through several methods:
- Microfossils: Microscopic structures that may preserve the shapes of ancient organisms.
- Stromatolites: Layered structures commonly associated with microbial communities.
- Carbon isotopes: Chemical signatures that can indicate biological carbon processing.
- Sedimentary structures: Geological features that may preserve evidence of ancient microbial activity.
Not every proposed ancient fossil is universally accepted as biological. Some mineral structures can resemble fossilized microorganisms, making careful examination essential.
Nevertheless, multiple lines of evidence indicate that microbial life was established on Earth during the Archean.
These early organisms transformed their environments through metabolic activity and established the biological foundations for subsequent evolutionary developments.
The Proterozoic Eon: A New Chapter in Earth's History
The Proterozoic Eon began approximately 2.5 billion years ago and continued until 538.8 million years ago. It was the longest of the three Precambrian eons and witnessed major transformations in Earth's atmosphere, oceans, continents, and living systems.
The Proterozoic is divided into three eras: the Paleoproterozoic, Mesoproterozoic, and Neoproterozoic.
Paleoproterozoic Era
The Paleoproterozoic lasted from approximately 2.5 to 1.6 billion years ago.
One of its most important developments was the Great Oxidation Event, during which atmospheric oxygen increased substantially. This transformation altered ocean chemistry and influenced the evolution of organisms that depended on oxygen. The era also witnessed major changes in continental development and several important glaciation events.
Mesoproterozoic Era
The Mesoproterozoic extended from approximately 1.6 to 1 billion years ago. During this interval, continental regions continued to evolve, and major landmasses assembled into the supercontinent Rodinia toward the later part of the era.
Evidence also indicates the continued development and diversification of eukaryotic organisms. However, the biological record remains incomplete, and the evolutionary relationships of some ancient fossils are uncertain.
Neoproterozoic Era
The Neoproterozoic lasted from approximately 1 billion to 538.8 million years ago. It included major environmental changes, extensive glaciations, and the development of increasingly complex multicellular life.
The Cryogenian Period, within the Neoproterozoic, is particularly famous for the Snowball Earth episodes. These events involved exceptionally extensive glaciation, although the exact extent of ice coverage and environmental conditions remains under scientific investigation.
The final part of the Neoproterozoic was the Ediacaran Period. It contains some of the earliest widely recognized fossils of large, complex multicellular organisms. The Proterozoic therefore represents a crucial stage in the transition from a predominantly microbial world toward ecosystems containing increasingly complex forms of life.
The Origin and Early Evolution of Life
The emergence of life is one of the most important and challenging questions in Earth science. Although the fossil record provides evidence of ancient organisms, the exact process through which nonliving chemistry developed into the first living systems remains unresolved.
Scientists use the term abiogenesis to describe the natural emergence of life from nonliving chemical processes.
Possible environments for life's origin
Several environments have been investigated as possible locations for the emergence of early life.
Hydrothermal vents are among the leading areas of interest. These systems occur where heated, mineral-rich water interacts with surrounding rocks, creating chemical gradients and sources of energy.
Other proposed environments include shallow pools, volcanic regions, mineral-rich surfaces, and coastal settings where cycles of wetting and drying may have concentrated important chemical compounds.
No single hypothesis has yet explained every step required to produce the first living system.
From chemistry to biological systems
Life requires several interconnected capabilities, including:
- A means of storing and transmitting biological information.
- Chemical reactions capable of obtaining and using energy.
- Structures that separate internal processes from the surrounding environment.
- A capacity for replication and evolutionary change.
Researchers investigate how simple organic molecules could have developed into more organized chemical systems capable of these functions.
The RNA world hypothesis proposes that RNA may have played an important early role because it can both store genetic information and participate in chemical reactions. Other models emphasize metabolism-first processes or interactions between genetic molecules and mineral surfaces.
These hypotheses are not necessarily mutually exclusive, and the actual origin of life may have involved several interacting stages.
Evidence from ancient rocks
Ancient rocks preserve chemical and structural evidence that may indicate biological activity. Carbon isotope patterns, microscopic structures, and sedimentary formations are particularly important.
Some evidence suggests that life existed by approximately 3.5 billion years ago. Older proposed evidence has attracted scientific debate because nonbiological processes can sometimes produce similar chemical or structural features.
The exact date and environment of life's origin therefore remain uncertain.
What is clear is that microbial life became established relatively early in Earth's history and eventually transformed the planet's oceans, atmosphere, and geological systems.
Cyanobacteria, Stromatolites, and the Evolution of Photosynthesis
Photosynthesis became one of the most influential biological processes in Earth's history. By capturing energy from sunlight and using it to drive chemical reactions, photosynthetic organisms transformed ecosystems and eventually altered atmospheric composition.
The evolution of oxygenic photosynthesis
Cyanobacteria are photosynthetic bacteria capable of using water as an electron donor and releasing oxygen as a by-product.
This process is known as oxygenic photosynthesis.
Unlike photosynthetic organisms that use alternative electron donors, oxygen-producing photosynthesis releases molecular oxygen into the surrounding environment.
Cyanobacteria played an important role in the development of this metabolic process, although the exact timing of its origin and early evolution remains an active research area.
Microbial mats and stromatolites
Many early microbial communities formed layered structures known as microbial mats.
These communities contained microorganisms that grew together on sediment surfaces. Their activity could trap, bind, or chemically precipitate sediment particles, gradually producing layered geological structures.
When these layers developed into recognizable sedimentary formations, they became known as stromatolites.
Stromatolites are particularly important because they preserve evidence of ancient microbial ecosystems. They occur in rocks from different geological periods and can still form in certain environments today.
However, stromatolites are not exclusive evidence of cyanobacteria. Different microbial communities and environmental processes can contribute to their formation.
Environmental consequences of photosynthesis
Oxygenic photosynthesis gradually influenced Earth's chemical environment.
Initially, much of the oxygen produced was consumed by reactions with dissolved iron, volcanic gases, and other reduced substances.
Consequently, oxygen did not immediately accumulate in the atmosphere. Instead, its environmental effects developed through complex interactions between biological activity, oceans, rocks, and atmospheric chemistry.
The evolution of oxygen-producing photosynthesis therefore established the biological process that eventually made large-scale atmospheric oxygenation possible.
The Great Oxidation Event: How Oxygen Transformed Earth
The Great Oxidation Event was a major environmental transformation during the Paleoproterozoic Era, approximately 2.4 billion years ago. It marked a substantial increase in atmospheric oxygen compared with the extremely low levels that characterized much of early Earth's history.
This event fundamentally changed the chemical conditions of the planet.
Why atmospheric oxygen increased
Oxygen-producing photosynthesis had already developed before the Great Oxidation Event.
However, the oxygen released by microorganisms initially reacted with substances in the oceans, atmosphere, and Earth's surface. These chemical reactions prevented large amounts of free oxygen from accumulating in the atmosphere.
Over geological time, changes in the balance between oxygen production and consumption allowed atmospheric oxygen to increase.
The precise combination of biological, geological, and environmental factors responsible for this transition remains an important research topic.
Formation of banded iron formations
One of the most recognizable geological records associated with ancient ocean chemistry is the banded iron formation.
These rocks contain alternating layers rich in iron-bearing minerals and silica.
In many ancient marine environments, dissolved iron reacted with oxygen or other oxidizing substances and formed iron-rich minerals that eventually accumulated as sediment.
Banded iron formations provide important evidence for changes in the chemical conditions of ancient oceans. Their formation involved complex interactions between ocean chemistry, biological processes, and geological conditions.
Effects on early life
The increase in atmospheric oxygen created new opportunities and challenges for living organisms.
For microorganisms that depended on oxygen, the changing environment eventually provided access to a powerful source of energy through aerobic respiration.
However, oxygen was harmful to many organisms adapted to oxygen-free conditions. Some may have survived in environments where oxygen remained scarce, including certain sediments and aquatic habitats.
The Great Oxidation Event did not instantly create today's oxygen-rich atmosphere. Oxygen levels remained substantially lower than modern values for much of the Proterozoic.
Nevertheless, this transformation established an important environmental foundation for the later evolution of increasingly complex organisms.
The Evolution of Eukaryotic Cells
One of the most significant developments in the history of life was the emergence of eukaryotic cells.
Unlike prokaryotic cells, which lack a membrane-bound nucleus, eukaryotic cells contain a nucleus that houses most of their genetic material. They also possess specialized internal structures called organelles.
Animals, plants, fungi, and many single-celled organisms are eukaryotes.
Differences between prokaryotic and eukaryotic cells
Prokaryotic organisms, including bacteria and archaea, generally have simpler cellular organization. Their genetic material is not enclosed within a membrane-bound nucleus.
Eukaryotic cells possess a more compartmentalized structure. Internal membranes separate different cellular processes, allowing specialized functions to operate within the same cell.
These characteristics provided new possibilities for cellular organization and evolutionary diversification.
The endosymbiotic theory
The endosymbiotic theory explains the evolutionary origin of certain eukaryotic organelles.
According to this theory, mitochondria originated from bacteria that entered into a long-term relationship with an ancestral host cell.
Rather than being digested, these bacteria became integrated into the host's cellular system. Over evolutionary time, they developed into organelles responsible for important aspects of cellular energy production.
Chloroplasts, the organelles responsible for photosynthesis in plants and algae, also have a bacterial evolutionary origin. They are generally understood to have descended from photosynthetic bacteria related to cyanobacteria.
Several observations support this theory:
- Mitochondria and chloroplasts contain their own DNA.
- They possess bacterial-like ribosomes.
- They reproduce through a process resembling bacterial division.
- Their membranes and genetic characteristics reflect their evolutionary history.
Why eukaryotic cells mattered
The emergence of eukaryotic cells represented a major increase in cellular complexity.
Their internal organization supported a wide range of biological functions and provided the foundation for the evolution of many different lineages.
Over time, eukaryotic organisms diversified into groups that included algae, fungi, animals, and plants.
Although the exact timing and sequence of early eukaryotic evolution remain under investigation, their emergence was a major step toward the biological diversity that would characterize later geological periods.
The Rise of Multicellular Life and the Ediacaran Biota
The evolution of multicellular organisms was another major development during the later Precambrian.
Multicellularity allowed groups of cells to work together, creating organisms with greater structural and functional complexity than most single-celled life.
However, multicellularity did not evolve only once. It appeared independently in several eukaryotic lineages, including the ancestors of animals, plants, and fungi.
Early multicellular organisms
Some ancient fossils preserve evidence of organisms composed of multiple cells or organized biological structures.
Algae and other eukaryotic groups provide important examples of multicellular evolution before the appearance of abundant animal fossils.
The development of multicellularity enabled greater specialization among cells. In some organisms, different cells became responsible for particular functions, such as support, reproduction, or nutrient processing.
These evolutionary changes helped create new forms of biological organization.
The Ediacaran biota
The Ediacaran Period, lasting approximately 635–538.8 million years ago, is especially important for its fossil record of large, soft-bodied organisms.
These fossils are collectively known as the Ediacaran biota.
Many Ediacaran organisms had unusual body shapes that differ considerably from most familiar modern animals.
Important examples include:
Dickinsonia: A flattened, oval-shaped organism with repeated body segments or ridges. Its precise biological relationships have been investigated extensively, and it is generally considered to represent an early animal or an animal-related organism.
Rangeomorphs: Organisms with branching, fractal-like body structures. Their biological classification remains uncertain, although they represent an important component of Ediacaran ecosystems.
Charnia: A fossil organism with a branching, frond-like appearance, commonly associated with the Ediacaran fossil record.
These organisms lived in marine environments and are often preserved through impressions in sedimentary rocks.
The transition toward the Cambrian
By the end of the Ediacaran, marine ecosystems contained a growing variety of complex organisms.
The Cambrian Period subsequently witnessed a major expansion in the diversity and fossil visibility of animal groups, including organisms with mineralized skeletons and more readily preserved body structures.
The transition was not an instantaneous appearance of all animal life. Evolutionary developments occurred before the Cambrian, while diversification continued during the period itself.
The Ediacaran biota therefore provides an essential record of life immediately preceding the Cambrian diversification.
Precambrian Climate, Supercontinents, and Major Geological Changes
Earth's climate and continental geography changed repeatedly throughout the Precambrian. These transformations influenced ocean circulation, atmospheric chemistry, geological processes, and the environments available to living organisms.
Early continental development
During the Archean and Proterozoic eons, continental crust developed and evolved through processes involving melting, volcanic activity, deformation, and crustal recycling.
Over geological time, smaller continental regions combined into larger landmasses.
These ancient continental configurations were different from the modern arrangement of Earth's continents.
Nuna and Rodinia
Two important Precambrian supercontinents were Nuna and Rodinia.
Nuna, also called Columbia in some scientific literature, is a proposed ancient supercontinent that assembled during the Paleoproterozoic and Mesoproterozoic.
Rodinia formed later, during the Proterozoic, and existed before its eventual fragmentation during the Neoproterozoic.
Reconstructions of these supercontinents rely on geological evidence, including matching rock formations, ancient mountain belts, paleomagnetic data, and the ages of mineral deposits.
Because the ancient continents have been extensively modified by later geological activity, their precise arrangements remain subjects of research.
Cryogenian glaciations and Snowball Earth
The Cryogenian Period, approximately 720–635 million years ago, included two major glaciation intervals commonly associated with Snowball Earth hypotheses.
These events are sometimes called the Sturtian and Marinoan glaciations.
Geological evidence indicates that ice sheets and glaciers extended into regions that are now associated with relatively low latitudes. This has led scientists to investigate the possibility of extremely extensive, potentially near-global glaciation.
The exact extent of ice coverage remains debated. Some models propose that much of the planet's surface became ice-covered, while others allow for areas of open water in certain environments.
These glaciations would have caused major environmental disruption, affecting oceans, atmospheric conditions, and biological habitats.
Environmental recovery and biological change
Following major glaciations, Earth's climate and ocean systems underwent significant changes.
Increased weathering, changes in ocean chemistry, and interactions between the atmosphere and geological systems may have contributed to environmental recovery.
The later Ediacaran Period witnessed the development of more complex marine communities, although the precise relationships between climate change and biological evolution remain an active area of research.
These events demonstrate that the Precambrian was not simply a long period of gradual biological development. It also involved major geological and climatic transformations that repeatedly changed the conditions of life.
Why Was the Precambrian Important?
The Precambrian established the geological, chemical, and biological foundations of the modern Earth. Nearly every major characteristic of the planet's later history can be connected to developments that occurred during this immense interval.
It created the planet's geological foundations
The formation of Earth's core, mantle, and crust established the internal structure responsible for many of the geological processes that continue today.
The development of continental crust, oceans, and early tectonic systems created the physical environments in which later geological and biological changes occurred.
It transformed Earth's atmosphere
The evolution of oxygen-producing photosynthesis and the Great Oxidation Event changed atmospheric chemistry.
Although oxygen levels remained variable and generally lower than modern conditions for much of the Proterozoic, the gradual increase in available oxygen helped establish conditions that supported increasingly complex forms of life.
It established the foundations of biological complexity
The Precambrian witnessed the emergence and diversification of microbial organisms, the evolution of eukaryotic cells, and the development of multicellular life.
These changes created the evolutionary foundations for the animals, plants, fungi, and other organisms that diversified during later geological periods.
It prepared the way for Cambrian diversification
By the end of the Precambrian, marine ecosystems contained a variety of complex organisms, including representatives of the Ediacaran biota.
The Cambrian Period followed with a major expansion in the diversity of animal fossils and the appearance of many recognizable animal groups.
However, this transition was part of a much longer evolutionary history that had already unfolded over billions of years.
The Precambrian is therefore not merely the period before familiar life appeared. It is the immense chapter during which Earth became a habitable planet and the foundations of biological complexity were established.
Conclusion
The Precambrian represents the longest and one of the most transformative chapters in Earth's history. From the formation of the early planet to the emergence of complex multicellular organisms, this interval witnessed fundamental changes in geology, atmospheric chemistry, climate, and biological evolution.
During the Hadean, Earth developed its internal structure and early surface environments. The Archean witnessed the establishment of ancient oceans, continental foundations, and microbial life. The Proterozoic brought major atmospheric changes, the evolution of eukaryotic cells, and the appearance of increasingly complex multicellular organisms.
These developments were interconnected. Geological processes shaped habitats, microorganisms changed ocean and atmospheric chemistry, and environmental transformations influenced the opportunities available for biological evolution.
By the end of the Precambrian, Earth had developed many of the conditions required for the diversification of complex life during the Phanerozoic Eon.
Understanding this immense interval is essential to understanding not only the origins of life but also how the modern planet became the world we recognize today.
The Precambrian refers to the entire span of Earth's history before the Cambrian Period. It includes the formation of Earth, the development of early oceans and continents, the emergence of life, and the evolution of increasingly complex organisms.
The Precambrian lasted approximately 4 billion years, beginning around 4.567 billion years ago and ending approximately 538.8 million years ago. It represents nearly 88% of Earth's geological history.
The Precambrian consists of three major eons: the Hadean, Archean, and Proterozoic. The Hadean covers Earth's earliest development, the Archean includes the establishment of early oceans and microbial life, and the Proterozoic witnessed major atmospheric and biological transformations.
The earliest known life was microbial, consisting of simple organisms without complex cellular structures. Bacteria and archaea are examples of these microorganisms. The exact origin and identity of the first living organisms remain uncertain because the earliest biological evidence is incomplete.
The Phanerozoic Eon followed the Precambrian, beginning approximately 538.8 million years ago with the Cambrian Period. This eon is characterized by a much richer fossil record of diverse animal, plant, and other complex life forms.