Where do we come from? Imagine the possibility life may have come from the fertile womb of the Universe to Earth as a tiny hitchhiking alien, using a meteor as a spacecraft. Anaxagorus, an ancient Greek, first proposed the theory that the seeds of life are spread throughout the universe. A science for discovering the foundations of life needs a theory ÃƒÆ’Ã‚Â¢ÃƒÂ¢Ã¢â‚¬Å¡Ã‚Â¬“ a biological Big Bang. One current theory has emerged from astrobiology, the science that searches for life in the universe. It is a candidate to replace the old concept that life arose on Earth in “primordial soup.”
Panspermia alleges that life exists and is distributed uniformly through the universe in the form of amino acids, microbes, germs, and spores. If life arose extraterrestrially, our planet is not a closed-system. The fossil evidence shows life took root on Earth as soon as possible, once the heavy bombardment period subsided, the planet cooled and water formed. Vulcanism and space debris made it inhospitable to life for the first half a billion years of planetary existence.
This “seed of life” can travel between worlds by natural means, such as ballistic impact, meteorites, and comets. Intergalactic space may be permeated with cosmic dust and microbes. Evidence shows they could survive the hard-core radiation and the near-absolute cold of deep space. Some researchers (Hoyle and Wickramasinghe, 2000) believe these “seeds” of life are still raining down on us all the time, affirming our cosmic ancestry.
Four billion years ago there was no DNA. It is widely believed that our DNA/protein based cells are derived from an earlier world based on RNA, which can both replicate information and be a catalyst for chemical or metabolic processes. In the prebiotic era, self-assembling RNA was both the genetic and catalytic basis. The simple genome resided in the RNA ÃƒÆ’Ã‚Â¢ÃƒÂ¢Ã¢â‚¬Å¡Ã‚Â¬“ a single circular chromosome.
We still don’t know how RNA arose in the first place (Poole, 1998). Perhaps it arose from some simpler self-replicating molecule. The evolutionary path from the RNA world led to the most primitive organisms, prokaryotes (bacteria and archaea) and eukaryotes (single-celled organisms).
Neither variety of primitive organism is a complete cell, but even prokaryotes have some free-floating DNA and ribosomes to make protein. Ribosomes “read” the genetic information and make whatever the cell needs. They possibly existed longer than 3.55 billion years ago, as their fossils and carbon deposits may indicate. Even exponents of competing theories of the origin of life agree ribosomes are at least 2.7 billion years old (Copley, 2003).
For 500 million years there were only RNA-based organisms. Primitive life could exist in hostile surroundings, with extreme heat, acidity, or with no oxygen or even light. Latest findings show this lifeform descends deep within the crust of our planet, and perhaps others. It seems life is not so fragile after all, but hearty and robust. The womb of our Universe is fertile, rather than hostile to life.
How life took a quantum leap into the world that eventually manifested human life is still a mystery. To call it life, you need a cell with both a nucleus and containing membrane. The mystery is written in the cells and molecules of all the life that still surrounds us.
The eukaryotes evolved in complexity, developing cellular characteristics. Arguably, there are fossils 3.8 billion years old that have structural molecules, ribosomes, and protein-synthesizing machinery. Proteins make the molecules for the “blueprint” molecule DNA possible. The stable DNA molecule became the genome carrier.