Earth’s Early History Section 19.3

The Early Earth Earth formed through collision of a series of cosmic debris 4.2 billion years ago it cooled enough for rocks to form Early atmosphere had little to no oxygen, Largely CO2, water vapor and nitrogen Very inhospitable!

The first organic molecules It was shown by Miller and Urey in 1953 that Earth’s early primitive atmosphere was capable of creating organic molecules – they created 21 amino acids! Although the composition for the Early Earth that they used was incorrect, but the general idea was correct

How do you go from organic molecules to living cells? A lot remains unknown about this step Evidence suggests 200 – 300 million years after the first rocks formed, cells similar to bacteria were common – how? Scientists have shown that under the correct conditions, large organic molecules can form protenoid bubbles called microspheres Not cells – but have some characteristics of cells Thought to acquire characteristics of living cells 3.8 billion years ago

Where did DNA and RNA come from? No one is sure, but a number of hypotheses exist RNA is thought to be able to form from simple organic molecules Based on experimental evidence RNA is very versatile – lead to the RNA world hypothesis RNA existed before DNA, and lead to direct DNA directed protein synthesis

Where did the Oxygen come from? First fossils that resemble bacteria come from 3.5 billion years ago Evolved in the absence of oxygen 2.2 billion years ago, photosynthetic organisms evolved Change the chemistry of the atmosphere Cooled the Earth Oxygen reacted with Iron in the Oceans – precipitating out Atmospheric composition changed over time – became toxic to many early organisms on Earth - adapt or die!

When did Eukaryotic cells originate? Proposed that 2 billion years, some ancient prokaryotes began to evolve internal cell membranes Endosymbiotic theory: prokaryotic cells entered these prokaryotes with an internal memberane Intruders did not infect or be digested, instead they began living within larger cells A symbiotic relationship developed between primitive eukaryotic cells and the prokaryotic cells within them Proposed due to similarities identified between membranes of chloroplasts and mitochondria and prokaryotes

Endosymbiotic theory explained

Modern Evidence DNA evidence shows that DNA from bacteria and DNA from mitochondria and chloroplasts are similar All have ribosomes of a similar structure and size They all reproduce by binary fission - division by mitosis All together these features suggest a common ancestor!

Sexual reproduction Sexual reproduction increases genetic variation and therefore sped yo evolutionary change Asexual reproduction leads to a direct genetic copy Genetic variation is restricted to mutations In sexual reproduction, genetic material comes from two parents Increased variation and increased chance of adapting to new or changing environmental conditions Multicellular organisms evolved a few hundred million years after the evolution of sexual reproduction Adaptive radiation lead to great diversity

Key points Earth’s original atmosphere was a harsh environment that could not support life as we know it The exact origin of life is unclear, but it is though to stem from the production of organic molecules such as proteins and RNA, leading to the formation of protenoid microspheres Photosynthetic bacteria lead to the production of an oxygen rich environment 2.2 billion years ago Eukaryotic cells developed through endosymbiotic theory – the ingestion of prokaryotes by a slightly altered prokaryote Sexual reproduction has lead to increased amounts of genetic variation and evolution