Objective: The objective of this module is examine what the fossil record can and can not tell us about the origin of life on earth. As you will see, scientific theories on the origin of life call upon the fossil record as well as modern biology and chemistry for data and insights. The evidence comes from both direct (e.g. fossils) and indirect (e.g., laboratory) sources.

BACKGROUND: In order to begin to look for the first signs of life in the fossil record, we have to define what life is. WHAT IS LIFE? Is it the presence of a metabolism that can provide for short term survival? If this is the case, then paleontologists looking for evidence of the earliest life, would want to search the geologic record for remnant chemical signals indicative of metabolism.

If life is better defined as the organic ability to reproduce for long term survival, then paleontologists looking for evidence of the earliest life on earth, would want to search for collections of small fossils that have the same features (....with the inference that they are reproducing themselves).

LOOKING FOR SIGNS OF ANCIENT LIFE....

Recently, the problem of defining what life is.. and what kind of fossil record early life would leave has become an issue for scientists studying the potential for life on Mars. Geochemists studying Martian meteorites are posing similar questions as their paleontologist counterparts. Take a tour through the Mars Life site linked below. It describes what characteristics basic forms of life might have. It also provides a good background on experiments that have been conducted to initiate life through combinations of basic chemicals and energy (e.g., electricity).

Martian Life.

Several chemists in the 1920's proposed that the prehistoric Landscape of the Earth (Oparin, 1924; Haldane, 1929) was likely comprised of methane, ammonium, water, hydrogen sulfide etc. This is the classic "Primordial Soup". They suggested that it was within this chemical environment that life could get its start.

Putting the Oparin-Haldane suggestions to the test, Miller and Urey designed experiments to replicate the primordial soup conditions. Their apparatus (below) and experiment yielded 15 of the 20 amino acids which form proteins. (READ and SEE MORE)

This was a pretty impressive result. Subsequently, other scientists were able to generate more complex organic compounds such as Oligopeptides and Polypeptides (proteinoids). Scientists were even able to produce proteinoid microspheres (Fox, 1965). Protective spheres would be essential to the development of complicated genetic molecules.

So in the lab, scientists could demonstrate many of the first steps towards life, however, the step from abiotic chemical processes to actual life is not something that could be easily duplicated in the lab. WHY?....

That next step... the formation of nucleic bases, which in combination bear information on nucleic acids such as DNA and RNA, is expected to have been a long evolutionary phenomena (millions of years of chemical "experiments"). The modern day experiment to duplicate this process would itself take millions of years! There is no early fossil record of DNA development to help us out.

Origin of Genetic Diversity

So what is this DNA... the Building block of life? For a short primer on genetics read the first page of the genetics primer from the Human Genome Project. When you are finished you should have a better understanding of the following:

Base Pairs= basic molecular units

DNA= strands of base pairs

Gene= portions of DNA that produce a recognizable effect or trait, differ in importance

Chromosomes= condensed DNA occurring in pairs with specific numbers of each species

Genetic Variation= idea that different organisms/individuals vary in their genetic structure

HUMAN GENOME PROJECT: GENETICS PRIMER

II. DIRECT INFORMATION ABOUT EARLY CELLULAR EVOLUTION:

Indirect information from modern biology and chemistry provides a good idea of how basic amino acids may have been formed on the way towards life. However, the means by which genetic material developed from such things as proteinoids has not been (and likely will not) duplicated in the lab.

Another important step that life is suggested to have taken early on was to employ Endosymbiosis, where smaller individual organisms joined together to form a better functioning cell (e.g., chloroplasts into a plant cell). They are thought to have done this for the mutual benefits of the union. (show me more!)

The next pieces of evidence about the origin of life come from the fossil record itself.

What are the Oldest Fossils and what do they reveal about the Timing and Character of Earth's Earliest Life?

There are three areas Akilia Island (Southern West Greenland), ISUA (Western Greenland), and Pilbara (Western Australia) that yield fossils greater than 3.5 billion years old.

The Akilia Island evidence was found in banded iron deposits and shows the concurrence of carbon intergrowths and apatite. The suggestion is that this combination is chemical evidence for and characteristic of metabolism. Note:There are no actual fossil organisms showing structure. Likewise, ISUA rocks contain Chemical Trace Fossils (e.g., Polycyclic Aromatic Hydrocarbons) from 3.8 billion ago also suggesting the evolution of metabolism

Rocks from Pilbara, Australia have actual Bacteria fossils and chemical traces dating back 3.5 billion ago!

Take a look at Bacteria fossils

The next steps for life after the development of bacteria have a good fossil record and are summarized in the the DAWN of LIFE EXHIBIT. The main steps include:

1. Evolution of the Eukaryotic Cells (those with a nucleus such as the higher plants and animals)

2. Rise of the Metazoans (the grouping of cells and cellular specialization within colonies)

Important examples are the Vendian Age Faunas (about 670-550 million years ago) from the Ediacara Hills of Australia.

3. Origin of Hardparts A major breakthrough in the structure of animal design was when their physiology adapted and evolved the ability to biomineralize such that organisms could secrete a hard skeleton. This breakthrough was rather swift and across many groups.

Major chemical types for skeletons: Carbonate, Phosphate, and Silica.