The Road to Advanced Life

Key points: Steps in evolution: nucleated cells, sex, biological switches; Cambrian Explosion; first vertebrates

Conditions on the early earth should have been favorable for the evolution of more complex life and even animals. But it didn't happen! For 1.5 - 2 billion years, the only life forms we can find are very similar to the first primitive bacteria and stromatolites pictured above.

Then the first cells with distinct nuclei appear (picture from   http://www.sp.uconn.edu/~terry/ images). (technically, non-nuclear cells are prokaryotes and nuclear ones are eukaryotes). It is thought that these represent the merger of two cells. It may have started by living together -- two cell types found they could make life much easier and pleasant for each other. Finally, they became so attached that they got married -- one engulfed the other, so they would stay together forever after. Nucleated cells have a tremendous advantage -- the nucleus need not carry out all the functions to survive in the outside world, like keeping the world out, and hence is free to become specialized -- for example, the DNA that encodes the reproduction is now stored in cell nuclei. In the prokaryote, the DNA is in a single chromosome that just "floats" in the cell. We have come to believe that eukaryotes should be considered as cooperative symbiotic colonies of cells, with the nucleus and other structures such as chloroplasts representing forms of prokaryote that have "banded together" to allow specialization and a more complex set of capabilities.

Here is a true "living fossil' -- the primitive eukaryote type cell that causes guardia!

Nucleated cells could be much more efficient in operating on their own, and more importantly their ability to specialize made it advantageous for them to associate with each other into multi-cellular organisms where different cells have different specializations.

About 1 billion years ago, there was another advance - sex. Life forms appeared where the full set of DNA for reproduction had to be drawn from two separate organisms, female and male. The mixing of genetic characteristics among organisms as a result of sexual reproduction suddenly increased the diversity of life immensely. Look at the examples below to understand why:

	Four individuals at the top have minor variations in their X and Z chromosomes that carry the code for their reproduction. Because they do not exchange genetic matter to reproduce (i.e., do not "have sex"), their offspring are genetically identical to the parents, and even after many generations have passed there are only four genetically distinct types.
	Four individuals at the top start with the same set of X and Z chromosomes as in the case above. However, these parents reproduce sexually (exchanging genetic material), so after many generations their descendents show all possible mixtures of the X and Z chromosomes -- even in this simple case, there are 16 distinct genetic types.

Thus, when mutations occurred, they resulted in a huge variety of organisms. The fittest survived, leading to rapid improvement in the ability of the plants and animals to survive and compete. Thus, sexual reproduction by specialized, eukaryote cells increased the rate of evolution toward complex organisms.

Plants and a few forms of  simple animals began to evolve. (left & right, G. Rieke, National Museum of Natural History; center, Dawkins, The Ancestor's Tale, http://macroevolution.narod.ru/ancestor/ancestor.htm),

These plants and animals were built by making a series of slightly modified copies of a small number of biological designs. This type of construction was another huge advance, and remains the way plants and animals develop today. The development process is controlled by biological "switches" that regulate the way large numbers of cells grow and organize themselves. As a result, evolution could proceed - a change in a switch is sufficient to produce a coherent, new form of organism.

Still, not much happened, until just over 500 million years ago there was a sudden appearance of complex multi-cellular plants and animals in the warm seas of the Cambrian era. The rapid increase in the varieties of life entering the Cambrian era is called the "Cambrian Explosion" - shown to the left as the number of biological "orders" in the fossil record, going from about 610 million to about 525 million years before the present (mybp) from Alan Kazlev and Toby White, http://www.palaeos.com/Paleozoic/Cambrian/Cambrian.htm

Compare these two re-creations of the Cambrian sea floor below with the one above for the Vendian/Ediacaran era. The Cambrian reconstruction to the right is dominated by the super-predator anomalocaris, but with a huge variety of other animal types. For the first time, predators hunted other animals that protected themselves with shells, spines, and other devices. (left, G. Rieke, Nat. Museum Nat. Hist; right http://paleobiology.si.edu/burgess/cambrianWorld.html; Royal Tyrrell Museum, picture by G. Rieke)

Reconstruction of anomalocaris, the dominant predator that grew up to two feet in length (picture by G. Rieke, Royal Tyrrell museum)	This awesome hunter (4 - 6 times larger than any other animal) swam the seas searching for food, which it captured with its claws and pulled into a round mouth lined with teeth. animation from D. Quinn, Paleoindustrial,  http://www.paleoindustrial.com/Anomalocaris%20-%20Swimming.htm.

These specimens of other creatures (typically a few inches long) are based on fossils from the Burgess Shale, in the Canadian Rockies on the border between British Columbia and Alberta. (top two images from http://home.entouch.net/dmd/cambevol.htm; Opabinia drawing from The Ancestor's Tale; tribolite by G. Rieke at the Burgess Shale)

Wiwaxia	Opabinia
Halucigenia	Trilobite

The oldest known vertebrates are small (1 inch long) fish-like chordates from the early Cambrian. (from Richard Dawkins, The Ancestor's Tale, http://macroevolution.narod.ru/ancestor/ancestor.htm), also the Halucigenia, both from China Changjiang shale deposits (rather than Burgess)	(from Stephen Greb through Kentucky Geological Survey, http://www.uky.edu/KGS/education/Imgcambrian.htm

This Aysheaia from the Burgess Shale is an example of an onychophoran, a family of animals that has been less successful than arthropods (insects, spiders)  or vertebrates. (from Living Landscape  http://www.livinglandscapes.bc.ca/cbasin/fossils/fig6.html)	This velvet worm, however, is a surviving example. (from Encyclopedia Brittanica Concies, http://concise.britannica.com/ebc/art-6767/Onychophoran)

The Cambrian era ended with a mass extinction, for which we are unsure of the cause.

Nonetheless, complex multi-cellular animals and plants continued to evolve, with intelligent life emerging only 500 million years later! An overview can be found in  http://www.ucmp.berkeley.edu/cambrian/camb.html and http://www.peripatus.gen.nz/Paleontology/Cambrian.html.

This narrative suggests that forming "life" in the form of cyanobacteria and stromatolites is not equivalent to forming complex organisms with intelligence -- from the time intervals, it would appear that the critical and difficult steps may have been the formation of cells with nuclei (eukaryotes) and then the breakthrough to complex animals.

Test your understanding before going on

God gives life to Adam, on the ceiling of the Sistine Chapel, by Michelangelo.		A comet rips the earth apart in this 19th century cartoon, (From C. Chapman, http://www.boulder.swri.edu/clark/chance/23oldcom.jpg)
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