Describe some of the hypotheses on the origin of life and the characteristics of early prokaryotes, eukaryotes, and multicelled organisms.
The oldest fossils provide evidence that life on Earth was established at least 3.5 billion years ago (Figure 1). Microscopic fossils similar to modern cyanobacteria have been found in silica-rich chert deposits worldwide. Notable examples include southern Africa, where rocks date to more than 3.1 billion years, and the Lake Superior region of western Ontario and northern Minnesota, where the Gunflint Chert contains some fossils older than 2 billion years. Chemical traces of organic matter in even older rocks have led paleontologists to conclude that life may have existed as early as 3.8 billion years ago.
Origin of Life
How did life begin? This question sparks considerable debate, and hypotheses abound. Requirements for life, assuming the presence of a hospitable environment, include the chemical raw materials that are found in essential molecules such as proteins. Proteins are made from organic compounds called amino acids. The first amino acids may have been synthesized from methane and ammonia, both of which were plentiful in Earth’s primitive atmosphere. Some scientists suggest that these gases could have been easily reorganized into useful organic molecules by ultraviolet light. Others consider lightning to have been the impetus, as the well-known experiments conducted by biochemists Stanley Miller and Harold Urey attempted to demonstrate.
Still other researchers suggest that amino acids arrived “ready-made,” delivered by asteroids or comets that collided with a young Earth. Evidence for this hypothesis comes from a group of meteorites, called carbonaceous chrondrites, which contain amino acid–like organic compounds.
Yet another hypothesis proposes that the organic material needed for life came from the methane and hydrogen sulfide that spews from deep-sea hydrothermal vents (black smokers). It is also possible that life originated in hot springs similar to those found in Yellowstone National Park.
Earth’s First Life: Prokaryotes
Regardless of where or how life originated, it is clear that the journey from “then” to “now” involved change (Figure 1). The first known organisms were simple single-cell bacteria called prokaryotes, which means their genetic material (DNA) is not separated from the rest of the cell by a nucleus. Because oxygen was largely absent from Earth’s early atmosphere and oceans, the first organisms employed anaerobic (without oxygen) metabolism to extract energy from “food.” Their food source was likely organic molecules in their surroundings, but that supply was very limited. Later, bacteria evolved that used solar energy to synthesize organic compounds (sugars).
This event was an important turning point in biological evolution: For the first time, organisms had the ability to produce food for themselves as well as for other life-forms.
Recall that photosynthesis by ancient cyanobacteria, a type of prokaryote, contributed to the gradual rise in the level of oxygen, first in the ocean and later in the atmosphere.
It was these early organisms, which began to inhabit Earth 3.5 billion years ago, that dramatically transformed our planet. Fossil evidence for the existence of these microscopic bacteria includes distinctively layered mats, called stromatolites, composed of slimy material secreted by these organisms, along with trapped sediments (Figure 2A).
What is known about these ancient fossils comes mainly from the study of modern stromatolites like those found in Shark Bay, Australia (Figure 2B).
Today’s stromatolites look like stubby pillars built as microbes slowly move upward to avoid being buried by the sediment that is continually deposited on them.
Evolution of Eukaryotes
The oldest fossils of more advanced organisms, called eukaryotes, are about 2.1 billion years old. The first eukaryotes were microscopic, single-cell organisms, but unlike prokaryotes, eukaryotes contain nuclei. This distinctive cellular structure is what all multicellular organisms that now inhabit our planet—trees, birds, fish, reptiles, and humans—have in common.
During much of the Precambrian, life consisted exclusively of single-celled organisms. It wasn’t until perhaps 1.2 billion years ago that multicelled eukaryotes evolved. Green algae, one of the first multicelled organisms, contained chloroplasts (used in photosynthesis) and were the likely ancestors of modern plants. The first primitive marine animals did not appear until somewhat later, perhaps 600 million years ago (Figure 3).
Fossil evidence suggests that organic evolution progressed at an excruciatingly slow pace until the end of the Precambrian. At that time, Earth’s continents were largely barren and the oceans were populated mainly with tiny organisms, many too small to be seen with the naked eye. Nevertheless, the stage was set for the evolution of larger and more complex plants and animals.