The Fossil Record

The Fossil Record 

A Brief History of Life 

Based on laboratory experiments conducted in the 1950s, researchers speculated that reactions in concentrated “soups” of chemicals that formed when seawater evaporated in shallow, coastal pools led to the formation of the earliest protein-like organic chemicals (“proto-life”). More recent studies suggest, instead, that such reactions took place in warm groundwater beneath the Earth’s surface or at hydrothermal vents on the sea floor. While the nature of proto-life remains a mystery, an image of early life has begun to take shape, based on detailed analysis of the oldest sedimentary rocks. The fossil record defines the subsequent long-term record of life’s evolution on planet Earth. And, of course, that record is more complete in younger strata.
Archaea and Bacteria fossils appear in rocks as old as about 3.7 billion years. For the next billion years or so of life history, cells of these organisms were the only types of life on Earth. Then, about 2.5 Ga, organisms of the Protista kingdom first appeared. Early multicellular organisms, shell-less invertebrates of the animal kingdom, and fungi came into existence at perhaps 1.0 to 1.5 Ga. Within each kingdom, life radiated (divided) into different phyla, and within each phylum, life radiated into different classes. The great variety of shelly invertebrate classes appeared a little over 530  million years ago, during an event called the Cambrian explosion, named for the geologic era in which it occurred. Many organisms that persist today appeared at different times since then. After the appearance of invertebrates came in sequence: fish, land plants, amphibians, reptiles, and finally birds and mammals. 

The "tree of life" symbolically illustrates relations among different domains of organisms on Earth. Archaea and Bacteria are both prokaryotic and don’t have nuclei. All other life forms are eukaryotic because they have cells with a nucleus.
Researchers have been working hard to understand the phylogeny (the evolutionary relationships) among organisms, using both the morphology of organisms and, more recently, the study of genetic material. Ideas about which groups radiated from which ancestors are shown in a chart called the “tree  of life”, or, more formally, the phylogenetic tree (figure above). Study of the DNA of different organisms is beginning to enable researchers to understand the relationship between molecular processes and evolutionary change.

Is the Fossil Record Complete? 

By some estimates, more than 250,000 species of eukaryote fossils have been collected and identified to date, by thousands of paleontolgists working on all continents during the past two centuries. These fossils define the framework of life’s evolution. But the record is not complete known fossils cannot account for every intermediate step in the evolution of every organism. Over the billions of years that life has existed there may have been 5 billion to 50 billion species of life in the three domains. Clearly, known fossils represent at most a tiny percentage of these species. Why is the record so incomplete? 
First, despite all the fossil-collecting efforts of the past two centuries, paleontologists have not even come close to sampling every cubic centimetre of sedimentary rock exposed on Earth. Just as biologists have not yet identified every living species of insect, paleontologists have not yet identified every species of fossil. New species and even genera of fossils continue to be discovered every year. 
Second, not all organisms are represented in the rock record, because not all organisms have a high preservation potential. As noted earlier, fossilization occurs only under special conditions, and thus only a minuscule fraction of the organisms that have lived on Earth have left a fossil record. There may be few, if any, fossils of a vast number of extinct species, so we have no way of knowing what they looked like or even that they ever existed. 
The sequence of sedimentary strata that exists on Earth does not account for every minute of time at every location since the formation of our planet. Sediments accumulate only in environments where conditions are appropriate for deposition and not for erosion sediments do not accumulate, for example, on dry plains or on steep mountain peaks, but do accumulate in the sea and in the floodplains and deltas of rivers. Because Earth’s climate changes through time and because the sea level rises and falls, certain locations on continents are sometimes sites of deposition and sometimes aren't, and on occasion are sites of erosion. Therefore, strata accumulate only episodically.
Credits: Stephen Marshak (Essentials of Geology)