33.2
What is Evolutionary History?
Around 4 billion years ago, Earth’s early atmosphere contained water vapor and gases released by volcanic eruptions. As the planet cooled, water vapor condensed into rain, making a significant contribution to the formation of early oceans.
Aquatic, single-celled organisms were the first living beings to inhabit Earth nearly 3.5 billion years ago. These early cells were prokaryotes - simple cells without a nucleus. Later, some prokaryotes, such as cyanobacteria, began using carbon dioxide to produce sugars through photosynthesis.
Oxygen, produced as a byproduct of photosynthesis, slowly accumulated in the atmosphere. Around 2.4 billion years ago, this increase led to the Great Oxidation Event, which raised atmospheric oxygen levels and changed Earth’s environment.
Evidence suggests that by around 2 billion years ago, some single-celled photosynthetic organisms may have begun colonizing moist terrestrial environments. Later, between 400 and 500 million years ago, multicellular organisms, particularly plants and fungi, established themselves on land, followed by animals.
Scientists reconstructed the evolutionary history of life on Earth—how life has changed over time—by studying fossils and building phylogenetic trees.
Fossils are preserved remains or imprints of organisms, such as bones or footprints. Together, they form a record of life’s history and provide strong evidence for evolution.
A phylogenetic tree is a diagram that shows evolutionary relationships among organisms. Its branching pattern shows how different groups share common ancestors.
For example, one might assume that whales and fish are closely related because both are aquatic animals with fins and tails used for swimming. However, anatomical comparisons show that modern whales share structural similarities with humans and other mammals, such as similar forelimbs.
Also, phylogenetic analyses suggest that whales share a more recent common ancestor with humans than with fish.
Phylogenetic analyses and fossil evidence show that whales evolved from terrestrial mammals—four-limbed tetrapods that moved from land back into the water. The similar body plans of whales and fish evolved independently through a process called convergent evolution.
Scientists record evolutionary history by analyzing fossil, morphological, and genetic data. The fossil record documents the history of life on Earth and provides evidence for evolution. However, both fossil and living organisms offer evidence that outlines Earth’s evolutionary history.
Phylogenetic trees illustrate the evolutionary relationships among these organisms. Scientists infer organisms’ common ancestry by evaluating shared morphological and genetic characteristics. Together, the fossil record and phylogenetic trees help scientists to reconstruct the evolutionary history of life on Earth.
According to evolutionary history, conditions on early Earth set the stage for life to begin. Nearly 4 billion years ago, atmospheric water vapor condensed into rain that filled the planet’s basins to form oceans. Consequently, as documented by fossil evidence, life on Earth began with the advent of unicellular life.
Scientists, such as astrobiologists, use this knowledge to research the potential for life on other planets. The presence of water is presumed to be a universally shared requirement for life. Water found on Mars, for example, suggests that life—most likely bacteria—may exist on that planet as well.
As conditions changed on Earth, organisms’ complexity and variety also changed. Oxygenation of Earth’s atmosphere paved the way for multicellular life and land colonization. Over time, countless species emerged (i.e., speciation) and perished (i.e., extinction) as Earth endured environmental shifts.
Throughout evolutionary time, species developed adaptations to better survive the Earth’s dynamic environment. Adaptations can arise from either a common ancestor or independently via convergent evolution. For example, a common ancestor gave rise to the shared forelimb structure of mammals, while the fins and tails of fish evolved independently from that of whales.
Evolutionary history describes how fossil and living species evolved since life’s emergence on Earth. Scientists aim to understand the environmental forces driving evolution as well as evolutionary relationships between organisms. Establishing Earth’s evolutionary history provides a framework for understanding the process and circumstances surrounding evolution.
Around 4 billion years ago, Earth’s early atmosphere contained water vapor and gases released by volcanic eruptions. As the planet cooled, water vapor condensed into rain, making a significant contribution to the formation of early oceans.
Aquatic, single-celled organisms were the first living beings to inhabit Earth nearly 3.5 billion years ago. These early cells were prokaryotes - simple cells without a nucleus. Later, some prokaryotes, such as cyanobacteria, began using carbon dioxide to produce sugars through photosynthesis.
Oxygen, produced as a byproduct of photosynthesis, slowly accumulated in the atmosphere. Around 2.4 billion years ago, this increase led to the Great Oxidation Event, which raised atmospheric oxygen levels and changed Earth’s environment.
Evidence suggests that by around 2 billion years ago, some single-celled photosynthetic organisms may have begun colonizing moist terrestrial environments. Later, between 400 and 500 million years ago, multicellular organisms, particularly plants and fungi, established themselves on land, followed by animals.
Scientists reconstructed the evolutionary history of life on Earth—how life has changed over time—by studying fossils and building phylogenetic trees.
Fossils are preserved remains or imprints of organisms, such as bones or footprints. Together, they form a record of life’s history and provide strong evidence for evolution.
A phylogenetic tree is a diagram that shows evolutionary relationships among organisms. Its branching pattern shows how different groups share common ancestors.
For example, one might assume that whales and fish are closely related because both are aquatic animals with fins and tails used for swimming. However, anatomical comparisons show that modern whales share structural similarities with humans and other mammals, such as similar forelimbs.
Also, phylogenetic analyses suggest that whales share a more recent common ancestor with humans than with fish.
Phylogenetic analyses and fossil evidence show that whales evolved from terrestrial mammals—four-limbed tetrapods that moved from land back into the water. The similar body plans of whales and fish evolved independently through a process called convergent evolution.
From Chapter 33:
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33.2 : What is Evolutionary History?
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33.1 : Conditions on Early Earth
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33.3 : The Fossil Record
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33.4 : The Evidence for Evolution
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33.5 : Phylogenetic Trees
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33.6 : Convergent Evolution
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33.7 : The Colonization of Land
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