The Origin and Evolution of life

The oldest life on Earth 1.Geological evidence 2.Biological evidence

Geologic evidence 1.Stromatolites 2.Microfossils 3.Isotopic evidence

Modern Stromatolite

3.5 billion year old Stromatolite

Discussion When we talk about the age of a rock, what are we actually measuring?

Radioactive dating Unstable parent isotopes decay at a constant rate to stable daughter isotopes. By measuring how much of the parent isotope is still present, and how much of the daughter isotope there is, we can calculate the age of the rock.

Rubidium 187Strontium Uranium 238Lead Plutonium 244Thorium Iodine 129Xenon Manganese 53Chromium Aluminum 26Magnesium parentdaughter½ life (millions of years)

Radioactive dating gives the time since the rock solidified. In the molten state daughter isotopes, which are lighter, can escape. Ages of rocks

Discussion Which is more likely to hold evidence of early life on earth, sedimentary rocks, or igneous rocks? Explain.

Discussion Sedimentary rocks cannot be dated using radioactive decay. Why not and how do we assign dates to them?

Discussion The oldest igneous rocks on earth can be dated back to 3.85 billion years. How can we claim then that the Earth formed 4.5 billion years ago? What happened to all the 4.5 billion year old rocks?

Age of the Solar System The oldest rocks on Earth are meteorites which fall from space and are about 4.55 billion years old. The oldest native Earth rocks are 3.85 billion years old.

Discussion If you wished to study the oldest rocks on Earth, would you find them on the continents or on the seafloor?

Zircon crystals Sandstone from Jack Hills Australia contains zircon which may have crystallized 4.4 billion years ago. Oxygen isotopes in the zircon indicates the presence of liquid surface water at this time.

Microfossil?

Carbon isotopes Carbon comes in two stable types: carbon 12 and carbon 13. Life prefers to use carbon 12 and rejects carbon 13.

Earliest evidence of life Even the oldest sedimentary rocks have enhanced levels of carbon 12.

Discussion Can you think of any reason why the Carbon 12 might be enhanced in these rocks without having life present 3.85 billion years ago?

Late heavy bombardment Based on radioactive dating of Apollo rock samples, the Moon experienced a period between 3.8 to 4.1 billion years ago during which it was subjected to many large asteroid impacts.

Discussion There is no record of the Earth being subjected to a late heavy bombardment like that of the Moon. Why do you think that is?

Formation of the Moon

Discussion Do you think Earth life could have survived the impact the formed the Moon? Explain.

Conclusion Life probably took less than 250 million years to evolve on Earth. The conversion for chemistry to biology must be easy under the right conditions.

Discussion This picture of an ancient stromatolite is returned tomorrow from the Mars rover. Astrobiologist rejoice claiming to have discovered proof of life on Mars. But biologists and geologists call the claim absurd and point out that non-biological processes can create similar structures. What do you think about this double standard?

Biological evidence All life on Earth probably started from a single stand of DNA which reproduced and evolved, becoming more complex with time.

DNA aging Assumptions: 1)All of Earth life comes for a single strand of DNA 2)Rate of replication errors is constant with time

How it works Non-coding DNA – DNA that is not used to make proteins, sometimes referred to as junk DNA. 1)Measure error rate from a particular species 2)Count the differences between DNA sequences of different species and divide by the error rate

Discussion Some researches believe that the error rate was much higher in the past than today. Why do you think that might be the case?

Hyperthermophiles The most primitive DNA studied to date is that of bacteria that live deep under the ocean around volcanic vents.

Black smoker

Discussion Does this mean that life first arose around volcanic vents? Explain.

Electric and Magnetic Waves

Circularly polarized light

Alanine

Life a low probability? The probability of a DNA molecule forming from random combinations of amino acids along with the very complex proteins needed for the DNA to replicate is very low.

Ribozymes Molecules of RNA that can encode information like DNA and act like proteins to catalyze chemical reactions. Might have been simpler precursor to DNA

HIV Retrovirus – transcribes RNA into DNA

How do we get from amino acids to a self replicating strand of RNA? Thioester peptide nucleic acids – spontaneously form RNA and DNA shaped molecules but do not self-replicate. tPNA

Evolution of life 1.Creation of amino acids 2.Creation of the first self replicating molecule – the random replicator 3.Enclosure of self replicating molecules within a vesicle or pre-cell

Evolution of life 4. pre-cells develop self-catalyzing RNA strands 5. Development of genetic code to transcribe RNA into DNA 6. Natural selection favors cells with DNA which can be quickly and accurately copied

Discussion What does it mean to say something is aerobic or anaerobic?

definitions Autotroph – gets carbon from carbon dioxide Heterotroph – gets carbon by eating other life

definitions Photoautotroph – autotroph that gets energy from sunlight Chemoautotroph – gets energy from chemical reactions involving inorganic chemicals

Discussion What do you think a chemoheterotroph is?

Earliest cells 1.Prokaryotes 2.Anaerobes 3.Chemoautotrophs

Discussion Why must the earliest life forms be prokaryotes? Why not eukaryotes?

Discussion Why must they have been anaerobic? Why not aerobic?

Discussion Why must they have been autotrophic, getting their carbon from CO 2 ?

Discussion Why do we think they were chemoautotrophes and not photoautotrophes?

Development of photosynthesis Prokaryotes evolved pigments to shield themselves from UV light Extra heat and energy from the absorbed light could be used