Overview

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.

Procedure

Around 4 billion years ago, Earth’s early atmosphere consisted of water vapor and gases produced by volcanic eruptions. As the planet cooled, the water vapor condensed to form rain, creating the Earth’s oceans. 

Aquatic, single-celled organisms emerged as the first living beings to inhabit Earth nearly 3.5 billion years ago. Some prokaryotes started fixing carbon dioxide to produce sugars—a process known as photosynthesis. 

Approximately 2 billion years ago, single-celled, photosynthetic organisms were the first to colonize wet, terrestrial environments. Oxygen, produced as a byproduct of photosynthesis, paved the way for Earth’s atmosphere to host more diverse, complex organisms.

Multicellular organisms—such as plants, fungi, and animals—began to populate Earth’s landmasses between 400-500 million years ago. 

Scientists reconstructed this and other accounts of life on Earth by analyzing the fossil record and developing phylogenetic trees.

Fossils are an organism’s preserved remains or imprints, such as bones or footprints. The fossil record is a collection of fossils that documents the history of life and provides evidence of evolution. 

A phylogenetic tree is a diagram that illustrates evolutionary relationships among organisms. The branching patterns of a phylogenetic tree indicate how organisms evolved from common ancestors.

Together, the fossil record and phylogenetic trees are effective tools for reconstructing evolutionary history. For example, one might assume a close evolutionary relationship between whales and fish, given that they both are aquatic animals with fins and a tail for swimming.

However, anatomical comparisons show that modern whales share structural similarities with humans and other mammals, such as similar forelimbs.

Furthermore, phylogenetic analyses suggest that whales indeed share a more recent common ancestor with humans rather than fish.

Scientists found evidence that modern whales evolved from a tetrapod, or four-limbed, organism that transitioned from land back into the water. The similar body plans of whales and fish instead evolved independently—a process called convergent evolution.

Evolutionary history aims to explain and understand how living organisms evolved since life emerged on Earth until the present day. The fossil record and phylogenetic trees serve as tools to help scientists reconstruct these accounts of life on Earth.