- Recognize that movement of continents over geologic time helps to explain why organisms are found where they are today
- Describe climatic conditions over geologic time (eg., Snowball Earth) and recognize how past conditions shape the traits of organisms over time (eg., oxygenic photosynthesis)
- Define adaptive radiations and their evolutionary importance
- Place specific origins of groups (nodes) and adaptive radiations (such as Cambrian Explosion, mammals) on the geologic time scale.
- When provided with a phylogenetic tree with specific evolutionary milestones, use evidence to place an unknown organism with specific, given traits on the phylogenetic tree of life and within the geologic time scale.
Continental Drift over Geologic Time Helps Explain Species Distributions
So far in our discussions of evolution of life on Earth in geologic time, we have emphasized the evolutionary adaptations within the tree of life that have led to evolution of new lineages resulting in our past and present diversity of life. But over geologic time, not only have species diversity and composition changed, but also the location of the continents themselves have shifted. Continental drift is the very gradual movement, assembly, and rifting of the crustal plates and their associated continents. This process means that when a taxon arose millions of years ago, it probably lived in a different location with respect to the equator and poles, and in a location that may have been connected physically with what are now separate continents.
Due to movement of tectonic plates, the continents move in a “supercontinent cycle,” or a periodic aggregation (coming together) and dispersal (breaking apart) over the course of 300-500 million years. When continents collide, the result is fewer and larger continents; a supercontinent results when all or most continents assemble together.
There have been multiple supercontinents throughout Earth’s history, but here we’ll briefly describe just two of them:
- Rodinia was a Precambrian (Proterozoic Eon) supercontinent that assembled approximately 1.2 BYA, and broke up approximately 750 MYA. Rodinia was located near the Earth’s equator, and its breakup around the end of the Proterozoic is thought to have contributed to climactic conditions that facilitated the rapid evolution of early multicellular life (more on this idea below).
- Pangea was Earth’s most recent supercontinent. It assembled approximately 335 MYA in the late Paleozoic, and began to break apart approximately 200 MYA during the early Mesozoic. Like Rodinia, Pangea was located near Earth’s equator. Assembly of Pangea dramatically affected evolution of life: prior to assembly of Pangea, the smaller continents located near the equator had wet and swampy conditions which favored massive lycophyte forests and amphibians; however, because larger continents tend to have drier climates in the continental interior; the assembly of Pangea resulted in increasingly arid conditions that favored gymnosperms and amniotes.
The short animation below shows the projected movement of continents, based on evidence from the magnetic rock record and other geological clues. As you view it, consider a specific group, such as reptiles, and when they arose and flourished globally.
Climate Change Over Time Shapes Evolution
Another abiotic (nonliving) factor that has dramatically impacted the evolution of life on Earth is climate change over time. We can infer a lot about climate conditions throughout Earth’s history based on evidence in the geologic record. Here we’ll describe three inter-related climate conditions in Earth’s early history: “Snowball Earth,” oxygenation of the atmosphere, and temperature changes over time.
Snowball Earth: Evidence from geology, the oxygenation of the atmosphere, and models of plate tectonics together provide evidence that the Earth was covered with glacial ice sometime preceding 600 MYA during the Proterozoic Eon. The hypothesis for this “Snowball Earth” ties together evidence of the following, not all of which are necessary for a global and long ice age to have occurred:
- Continents located primarily near the equator: Land at the equator reflects sunlight more effectively than open ocean, which maximizes the albedo effect (cooling due to reflected sunlight). While oceans absorb heat from the sun rather than reflect it, land at the equator actually reflects light and heat, preventing surface temperature from increasing. Rodinia was located at the equator prior to its breakup ~750 MYA.
- Excessive production of oxygen: As Rodinia broke up ~750 MYA, its separation created coastal environments with excess availability of mineral runoff into the oceans. Cyanobacteria in the oceans flourished as a result, producing oxygen in excess of the amount that could be sequestered by reactions in the ocean with iron. These reactions formed rock layers with alternating bands of iron-rich sediment. These so-called banded iron formations indicate that oxygen was present in the oceans or seas and reacting with dissolved iron to form iron oxides, which were heavy and sank to the ocean floor.
- Reduction of greenhouse gasses: Once iron and other reactants, like organic carbon, in the ocean became depleted, the excess oxygen eventually accumulated in the atmosphere, where it may have reacted with the “greenhouse gas” methane to form carbon dioxide. The subsequent reduction in methane would have reduced the ability of the planet to retain the reflected light and heat, allowing the surface to cool.
Oxygenation of the atmosphere: No other planet in our solar system has oxygen gas in the atmosphere. Oxygen is highly reactive, and quickly consumed by oxidation reactions. On Earth, oxygenic photosynthesis by cyanobacteria, green plants, and many protists continually replenishes the oxygen consumed by respiration and other oxidative processes (e.g., rusting of iron, weathering of rocks). Evolution of oxygenic photosynthesis changed the planet’s atmosphere over billions of years, and in turn caused radical changes in the evolution of life on Earth. The history of oxygen gas in the Earth’s atmosphere greatly influenced the history of life, as follows:
- Prior to the evolution of oxygenic photosynthesis in early cyanobacteria approximately 2.5 BYA during the late Archean, the early Earth had essentially no oxygen gas (O2) in the atmosphere and all life was anaerobic
- Beginning ~2.5 BYA at the start of the Proterozoic eon, early cyanobacteria produce O2 as a byproduct of oxygenic photosynthesis in the oceans. The O2 is absorbed in oceans and seabed rock by reacting with soluble iron and precipitated iron oxide (rust) from the oceans, generating banded iron formations. Oxygen dissolved in water causes mass extinction of many anaerobic organisms (“oxygen catastrophe”) and allows evolution of aerobic metabolism, which is energetically more efficient, and eventually enables the evolution of the first eukaryotes around 1.6-2 BYA
- In the mid Proterozoic eon, O2 starts to gas out of the oceans into the atmosphere and is absorbed by land surfaces and formation of ozone layer. The first multicellular eukaryotes evolve, including algaes and animals.
- In the late Proterozoic, O2 sinks become saturated and the gas accumulates in the atmosphere. Accumulation of oxygen in the atmosphere facilitates the Cambrian explosion of large and complex multicellular life ~542 MYA, initiating the Phanerozoic eon.
Temperature and sea level: The Earth has experienced large swings in global temperature, from the probable “Snowball Earth” in the Proterozoic to a fluctuation between hothouse and intermittent glaciation, shown below, in the Phanerozoic.
Multiple factors caused these large swings in global temperature, including changes in solar output, extreme volcanic activity, meteor strikes, and atmospheric greenhouse gases, such as carbon dioxide. Carbon dioxide levels are currently above 400 ppm, the highest it has been for at least the last 14 million years.
Changes in Earths continental arrangements and climate have dramatic impacts on evolution, including both extinctions and adaptive radiations. Adaptive radiations are periods of increasing biodiversity and rapid speciation in geologic time. (“Rapid” in geologic time means “millions of years.”) In a phylogenetic tree, an adaptive radiation often looks “tufty,” like the tip of a paintbrush, with lots of short bristles (branches) coming off a longer branch. There are several situations which can allow for an adaptive radiation to occur such as:
- When new ecological niches become available, which can occur after a major climate event like the oxygen revolution or after a mass extinction. An example is when all non-avian dinosaurs went extinct at the end-Cretaceous mass extinction, which allowed the surviving mammals to diversify into the niches that had previously been occupied by dinosaurs; the dinosaurs were no longer present to outcompete the mammals.
- In response to a key “evolutionary innovation” which is a novel (new) adaptive trait that in the environment. Examples include traits like jaws, multicellularity, wings, nectar in flowers, or flat molars (which allow for chewing) in mammals. Each of these opens up newly available resources to the individuals with the trait. For example, the evolution of jaws gave early gnathostomes the ability to bite other organisms, which opened up an array of available prey items.
We have previously discussed the Cambrian “explosion,” an adaptive radiation which led to an increase in biodiversity of multicellular organisms at the start of the Cambrian, 542 million years ago. Multicellular life appeared only several tens of millions of years before the start of the Cambrian, as enigmatic fossils (Ediacaran biota) exhibiting body plans unlike present-day animals. These early multicellular organisms largely disappeared and were replaced by Cambrian fauna, whose variety includes all of the body plans found in present-day animal phyla. The appearance of Cambrian fauna, although relatively quick in evolutionary time, actually spans millions of years; they did not all appear simultaneously, as the term “explosion” inaccurately implies.
The Cambrian species radiation also coincided with an increase in free oxygen to near-present day levels (more on that below), which matters because available oxygen allows for highly efficient metabolic energy use.
Origins and Radiations can be Mapped to Geologic Time
Reflect back on the diversity of species from this module, and consider when groups first arose and when adaptive radiations occurred in each group. For example, mammals arose in the Mesozoic but didn’t radiate (diversify) until the Cenozoic, after the dinosaurs’ extinction at the end-Cretaceous opened up new niches. If we map those radiations (denoted by the word “flourished” in the table below) to the geologic time scale, we can correlate major earth events with lineage diversification: the change in atmospheric oxygen over time, changes in global climate, the location of continental land masses through continental drift and associated sea-level changes.
To help you put this information together in context of other evolutionary events in the history of life on Earth, look back over the previous readings listed below and specifically focus on the section discussing evolutionary events in Geologic Time:
- Land Plant Evolution over Geologic Time
- Fungi Evolution over Geologic Time
- Invertebrate Evolution over Geologic Time
- Vertebrate Evolution over Geologic Time
This short PBS Eons video ties together a group’s proliferation or radiation with the major geologic periods, and gives some perspective on both how much we can understand about the history of life using fossils, and how much mystery might remain in the fossil record for the 99% of taxa that have gone extinct already.
Apply Your Knowledge to Place Unknown Organisms onto the Tree of Life
We began the module by learning how to read phylogenies. In the previous readings in this Biodiversity Module, we’ve noted that key innovations or adaptations arose in lineages and came to define the clade. For example, in the tetrapod phylogeny depicted at this link, all living tetrapods share the trait of five digits, with wrists and ankles turned forward for walking on land. If you were presented with a vertebrate specimen that also had these characteristics, you would logically label it as a tetrapod, and then use other additional characteristics to further categorize it and consider when it lived and what else flourished at that time. As the module wraps up, you now have the additional biodiversity knowledge to place organisms of unknown origin onto the phylogenetic tree of life and into the geologic time scale.
The Khan Academy video below on the Proterozoic Eon below reviews many of these ideas:
And here’s a PBS Eons video with an overview of oxygen, diversification, and extinction, which makes a nice bridge to the reading for next time on extinction: