READ: Gallery — How Did Life Begin and Change?

A closer look at the complexity of life and some of its important "mini-thresholds."

One way of looking at the development of life on Earth is to identify "mini-thresholds," times when life seems to have gotten distinctly more complex.

Scientists named the side-by-side columns of this hydrothermal vent The Brothers. Deep sea vents, or "black smokers" as they are often called, occur at oceanic ridges where new sea floor emerges from the Earth's mantle. Scientists believe that some of the earliest life on Earth may have formed around these vents when chemical rich material from within the Earth poured into the oceans. Today's "vent communities" include tube worms, a variety of crustaceans and special bacteria that convert chemicals into energy rather than using the Sun's light.

In this microscopic image of an Amoeba capturing a Paramecium, the single-celled eukaryotic Amoeba demonstrates remarkable purpose by using its pseudopods to engulf its prey. The Amoeba will change its shape to completely surround the Paramecium and will absorb its meal using organelles called vacuoles.

Cyanobacteria are also known as blue-green algae and their name comes from the Greek word kyanós for blue. This phylum of microorganisms often form large "colonies" and their highly visible blooms, like this one in the Pacific Ocean near Fiji, can be seen and photographed from far above the Earth's surface. Cyanobacteria have been a predominant life form on Earth for at least 2.8 billion years and their oxygen-producing photosynthesis increased oxygen levels in the atmosphere, encouraging further biodiversity.

The chloroplasts that conduct photosynthesis in the green plants that surround us are thought to have evolved from cyanobacteria by endosymbiosis. According to this theory, free-living bacteria were taken inside, or swallowed up, by cyanobacteria, forming new cells with specialized organelles that had the ability to conduct more complex functions. Chloroplasts capture light energy and carbon dioxide and transform them into energy-rich molecules like ATP and NADPH and, eventually, glucose.

One of the other things that happens when photosynthesis occurs is that a lot of carbon dioxide is transformed into oxygen. When green plants first started in with photosynthesis it altered the atmosphere, greatly increasing oxygen levels. Many species, unaccustomed to an oxygen-rich climate and unable to survive, perished in what scientists sometimes call "the Great Oxidation Event" or "the Oxygen Holocaust." Other life forms liked the new air and flourished.

English natural philosopher Robert Hooke was the first to use the word "cell" to describe the basic unit of life. He was examining a thin slice from a dead cork plant under a microscope and the walled compartments reminded him of the cells that monks of his time often lived in. Hooke published his description of cells and the membranes that separated them in a 1665 book called Micrographia, a work thought to be one of the first scientific bestsellers.

An earthworm has bunches of nerve cells, called ganglia (singular ganglion), that operate similar to brains, processing sensory information and sending instructions to the worm's other organs to control movement and other functions. This photograph shows the dorsal ganglion in a dissected earthworm, thought to be an example of an early stage in the evolution of brains.

Nervous system cells or nerve cells, shown here, are called neurons. Mammals can have hundreds of billions of neurons in their brains, all of them working together to process information and send signals to other parts of the body by "firing." Inside each neuron are many of the things found in other types of eukaryotic cells, such as mitochondria, a nucleus with DNA and other organelles.

The Tiktaalik roseae is considered a transitional fossil, or missing link, between fish and the first land-based animals of the late Devonian period. This illustration, based on fossils about 375 million years old, shows how the fishes' front fins evolved into the first arm-like legs, similar to those you might see on reptiles like crocodiles and critical to the new animal's movement on land.

Some of the earliest mammals were thought to be mouse-like animals that lived at the time of the dinosaurs and prospered when the K-T impact wiped out the dinosaurs. The fossil shown here is that of a Leptictidium auderiense, a now-extinct mammal that lived about 50 million years ago.