History of Life on Earth

Chapter 25: History of Life on Earth Synthesis of Organic Compounds on Early Earth

The Earth probably formed about 4. 6 billion years ago, and was bombarded with rocks and other material until about 3. 9 billion years ago.

The Earth then cooled, allowing for the formation of oceans. Scientists hypothesize the general atmosphere, or at least some regions, were naturally reducing environments, meaning that they added electrons to compounds.

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Activation energy provided by lightening or UV radiation may have been able to create organic compounds and amino acids, as demonstrated by a number of modern experiments.

Abiotic Synthesis of Macromolecules

Experiments have been done in which amino acid solutions in hot sand have formed polymers, but not true proteins. These polymers may have functioned as basic catalysts of some kind, however.

Protobionts

Cells have genetic material in the form of DNA and RNA, which they are also capable of replicating. Nothing like this has been generated spontaneously in lab experiments.

However, early structures called protobionts have had some of the capabilities associated with life. Experiments have spontaneously create protobionts, which are simple sphere of membrane that can perform simple metabolic and reproductive functions.

Note: phospholipids spontaneously form a bilayer, like the membrane that surrounds cells, so that part of the puzzle is easy to solve. Self-Replicating RNA and the Dawn of Natural Selection

Simple RNA structures called ribozymes can carry out basic chemical reactions and are even capable of replicating themselves. As ribozymes replicated themselves (with errors) protobionts could have developed internal collections of slightly different enzymes that formed a rudimentary metabolic system. The RNA in these early “cells” may have served as a template for the eventual creation of a DNA genome, which would have reduced the number of errors made during replication.

The Fossil Record

The fossil record gives a glimpse of life on Earth during different time periods and provides clues for evolutionary research. However, the fossil record also has significant gaps, but some are being filled by new discoveries.

How Rocks and Fossils are Dated

Fossils appear in individual sediment layers, which tell us the order that they were formed in but not an actual age in years. Scientists use radiometric dating to determine absolute ages.  Radiometric dating is based on the fact that some radioactive elements have predictable half-lives, or periods in which half of the substance will decay. If you know how much of a certain radioactive element an organism has when it died, such as carbon-14, you can determine its age by measuring the amount of carbon-14 remaining today. Older fossils are harder to date, but you can at least guess based on the age of fossils in the surrounding layers. The Origin of New Groups of Organisms The presence of certain bones, different kinds of teeth and other characteristics can help researchers make inferences about what an animal may been like while it was alive. Changes between similar fossils of different time periods also show the pace of evolutionary change.

The First Single-Celled Organisms

Scientists have found fossilized stromolites that are thought to have lived 3. 5 billion years ago – the earliest organisms discovered to date. Stromolites are mounds of prokaryotes that bind to their kin and other inorganic material.  Photosynthesis and the Oxygen Revolution 2. 7 billion years ago, there were probably cynobacteria in the ocean that used photosynthesis for energy and released oxygen in the process.

The oxygen that these bacteria released would have eventually begun reacting with iron, and finally escaped into the atmosphere as a gas.  This buildup of oxygen actually killed many prokaryotes, and provided a strong selective force in favor of cells that could use oxygen in their metabolism. The cells that survived the “oxygen revolution” are probably the ancestors of today’s aerobic organisms. The First Eukaryotes The earliest eukaryotes are thought to have lived around 2. 1 billion years ago. These early eukaryotes probably formed through endosymbiosis, in which they engulfed small, living, cells and developed a mutualistic (mutually beneficial) relationship with them.  The mitochondria, for example, may have been “formed” in this way. The mitochondria has a double membrane, maintains and replicates its own DNA and reproduce independently of the rest of the cell. The Origin of Multicellularity: As cells became more complex, they also came to exhibit greater diversity.

Multicellular structures also began to form. The Earliest Multicellular Eukaryotes The earliest multicellular eukaryotes probably lived around 1. 5 billion years ago. Earth had a few ice ages between 750-580 million years ago. Eukaryote diversity increased after the end of this period. The Cambrian Explosion  Scientists have found a whole bunch of animal fossils from the Cambrian Period (535-525 million years ago). While previous animals seem to have mostly been herbivores or filter-feeders, the animals that arose during the Cambrian Explosion had claws and armor. Recent evidence, however, suggests that some of these animals were living before the Cambrian Period, so maybe “explosion” isn’t the best term.

The Colonization of Land

Prokaryotes lived on land as long as 1 billion years ago, but larger organisms wouldn’t get there until around 500 million years ago. Plants, which often had mutually beneficial relationships with fungi, colonized the land and began developing specialized structures suited to life out of water. Arthropods, such as insects, also spread on to the land. Continental Drift Here’s something weird: the continents move. They might move really slowly, when you’re talking about millions of years, those tiny movements add up. This process, called continental drift, involves the movement of the Earth’s plates. The collision of two plates can form mountain ranges, cause earthquakes, and so forth.

Consequences of Continental Drift

Around 250 million years ago, most of the Earth’s land was concentrated into once massive continent called Pangaea. As the plates moved, climates changed (sometimes dramatically) and many species went extinct.

The separation of plates, in contrast, promotes allopatric speciation. These changes in the Earth’s geography help explain similarities between distant organisms, for example. Mass Extinctions

Species go extinct all the time, but there are also major events that are particularly important in evolutionary history – mass extinctions. The “Big Five” Mass Extinction Events. Here are a few stats involving fives for you: Over the past 500 million years  We’ve had 5 mass extinctions Each of which caused the deaths of more than 50% of the world’s species  In the Permian mass extinction, massive volcanic activity spewed lava over 1. 6 million square kilometers and released a huge amount of carbon dioxide, which may have warmed the planet and indirectly caused the deaths of many aquatic organisms.  In the Cretaceous mass extinction, which killed many of the dinosaurs, an asteroid probably struck the Earth. This would have created a huge cloud of debris that could block the sun and alter the planet’s climate rather significantly.

Such an impact may have created the Chicxulub crater in Mexico. Is a Sixth Mass Extinction Under Way? Humans haven’t killed as many species as the other mass extinctions did, but we’re killing them between 100-1,000 times faster than they normally die. And that could be a big problem.

Consequences of Mass Extinctions

It generally takes a few million years for the number of species on the Earth to rebound after a mass extinction. These events also generally wipe out species without regard to their fitness or environmental adaptations.

Adaptive Radiations

The diversity of organisms has increased in the last 250 million years, as populations adapt to new environmental conditions and undergo speciation. These adaptive radiations generally occur on a large scale after mass extinctions, which leave many ecological niches open.

Worldwide Adaptive Radiations

After the dinosaurs went extinct around 65. 5 million years ago, mammals underwent significant adaptive radiation, filling the roles that now-extinct species had occupied in individual ecosystems. Regional Adaptive Radiations Hawaii, which is far from any other continent, is a particularly stark example of adaptive radiation. There are hundreds of species on Hawaii that don’t exist anywhere else in the world.

Evolutionary Effects of Developmental Genes: Developmental patterns have also changed over time. Changes in Rate and Timing: One common developmental change is heterochrony, in which different parts of the body grow at different rates or at different times. Some organisms can undergo phetomorphosis, in which the adult form of the organism retains traits that previously had been confined to children. Basically, if human adults started looking like babies, we would have a phetomorphosis situation on our hands. Changes in Spatial Pattern: Changes in the homeotic genes, which control how and where individual body structures develop, could have led to the development of vertebrates and other organisms. The Evolution of Development: There are a few different ways that mutations can significantly influence body structure. Changes in Genes * Many organisms have similar genes that are nevertheless different enough to produce very different outcomes.

By identifying and testing each difference between the two genes, researchers can pinpoint the precise changes that alter the gene’s function. Changes in Gene Regulation: Sometimes changes in gene regulation, and thus gene expression, can alter an organism’s body structure. These changes can be localized to specific types of cells, and thus are less potentially dangerous than changes to the genome itself. Evolutionary Novelties: Evolution doesn’t proceed with a final goal in mind, and just involves slight changes from one generation to the next. Over time, simpler structures can become more complex and useful.

Structures can also develop into something that serves a totally different purpose than their original function. Evolutionary Trends: The problem with looking at evolutionary trends (such as “horses are getting bigger”), you’re examining a linear succession of different horse ancestors but rather a branched tree of ancestors that diverged in all different directions. However, natural selection also works on entire species. If speciation is the birth of a species and extinction is its death, natural selection could guide the development of these successive “generations” and thus create a trend.

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