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Gallery: How Did Life Begin and Change?

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


The Big History Project
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.

The Brothers

National Oceanic and Atmospheric Administration (NOAA)
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.

A Single-celled Predator

© Carolina Biological/Visuals Unlimited/Corbis
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.


Norman Kuring/NASA Ocean Color
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.


Kristian Peters
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.

Making Oxygen

The Big History Project
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.

The First Named Cell

An illustration by Robert Hooke
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.

Early Brains

© Carolina Biological/Visuals Unlimited/Corbis
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.

Brain Cells

© MedicalRF.com/Corbis
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.

Moving to Dry Land

National Science Foundation
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.

Mouse to Man?

Muséum National d'Histoire Naturelle
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.

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  • mr pink red style avatar for user WallAvi
    Isn't it a little odd that there is not a greater degree of bio diversity and differentiation in some species like canines, equine, porcine, bovines and humans.

    The diversity in canine, equine, bovine, and porcine are mostly due to mans intervention and relatively recent aren't they?

    The physical diversity and differentiation evidenced in the past and still surviving and present in some species over greater amounts of time go from one incredible extreme to another but not in others.

    Avian/birds : bee humming bird to ostrich to penguin.
    Feline: civet /cervical/ ocelot (not sure which is smallest) to saber tooth tiger
    Primates: marmoset to ape/gorilla
    Arachnids: patu marplesi to tarantula
    Rodents: pygmy jerboa to flying fruit bats
    Reptilian: dinosaur to leaf chameleons to cayman / thread snake to anaconda/python.
    Jellyfish: Irukandji to lions mane jelly fish

    Insects are in a class all by themselves not to mention fish and plants.

    Rocks have more real diversity and variety than humans do.

    I do realize that man is considered a primate....but shouldn't there be more species of humans and more physical diversity besides skin, eye and hair color? I'm not suggesting that it would have to be on a Mos Eisley cantina level of diversity but it seems like there should be more physical differences shouldn't there? After all there are numerous species types of marmosets. But only one species of humans.
    (3 votes)
    Default Khan Academy avatar avatar for user
    • piceratops ultimate style avatar for user Becky Cribdon
      We need to be careful how we compare the biodiversity of groups - the groups you've mentioned above are artificial, after all. We've put names on boxes, and not all boxes are of equal size or age. 'Reptile' includes every single animal which lays descended from something which laid eggs but is not a bird or mammal, so it contains a far bigger proportion of the vertebrate tree than a little box, like the genus Homo.

      The number of species in a genus is better comparison. A quick look on Wikipedia tells me that about fifteen species have been put in Homo, and the distinction between Homo and Australopithecus can be woolly, so some might say it's even more than that. As a biologist, I think fifteen is pretty normal.

      As for diversity within Homo sapiens, have you heard of population bottlenecks? This is where a population suddenly decreases by a large amount, so if it recovers, the amount of genetic diversity will be lower than before the bottleneck, basically because everyone is more closely related than before. There is evidence that we underwent one or a few bottlenecks at some point after our species emerged; even though the details aren't clear, we do have much lower genetic diversity than other apes. It's actually a bit surprising how much humans can look different, considering we've all got relatively similar DNA. We have much more physical diversity than you might expect!
      (10 votes)