Origins of Life

1. Origins of Life Jack Pickett/Kelley Kuhn

2. Write A definition and/or give examples

3. What does the Earth Tell us About its history? • There are many rock layers that are exposed for humans to see and study. (Have you ever driven through Monteagle?) • Across the world, these layers have been compared. Rock layers with the same fossils, appearance, and mineral content are assumed to have formed at the same time in earth’s history. • Based on the Law of Superposition, we assume that the rock layers that are the deepest are the oldest. • All of this information has been used to generate (drum roll please)…. THE GEOLOGIC TIME SCALE!

4. Yikes! Don’t worry…you do not have to memorize this!

5. Why is this pertinent… • Most of the assumptions that we make in Biology about the history of life are based on what we find in these rock layers. This is what we find in the fossil record: • There are no fossils in the oldest rock layers. • As you move up through the rock layers, you first find single-celled organisms, then simple invertebrates, and finally more complex organisms as you study some of the youngest rock layers. • This leads scientists to conclude: (write here the logical conclusion of these observations)

6. The main Limitation OF THE FOSSIL RECORD • The fossils in the fossil record do not show a steady gradient of change. In other words, there are gaps in the fossil record. • How do scientists address these gaps? • 1. It is actually pretty rare for fossils to form. You must have a specific set of conditions occur for a fossil to form. • 2. Soft tissue organisms (like worms for example) are far less likely to form fossils than organisms with a skeletal system. • 3. The entire rock record may not be visible for human study.

7. The most difficult question… • The hardest question for scientists to answer is this: HOW DID THE FIRST CELL ARISE? There are several aspects to this question: How did the biomolecules like protein and DNA arise? How did the first cell membrane form? How was early DNA replicated? Where did the first life arise on the planet? How come molecules in living organisms are “handed”? A few things we assume about the earliest cells: They were anaerobic. They were prokaryotic. They formed in water. Why do we make the above assumptions? Think about it a minute and be prepared to share.

8. Which led to a great biological irony… • LazzaroSpallanzani, Francesco Redi and Louis Pasteur established in the 18th and 19thcenturies that life arrives only from other living things…This is called biogenesis. (Pasteur said“Never will the doctrine of spontaneous generation recover from the mortal blow struck by this simple experiment.“) • Redi: maggotsand meat and cheesecloth • Spallanzani: killed microbes by boiling • Pasteur: swan-necked flask with milk • HOWEVER, modern scientists believe that the first cells on earth arose via abiogenesis (a fancy word for spontaneous generation).

9. How did biomolecules arise? • Every hear of the primordial soup? In Miller Urey apparatus, which was supposed to simulate the environment on early earth, amino acids were synthesized. • Problem: the best guess is that earth’s primitive atmosphere contained nitrogen (N2), Carbon dioxide (CO2), water vapor (H2O), and maybe some carbon monoxide (CO), but no methane or ammonia were present.

10. How did the first cell membranes form? • Phospholipids and similar molecules have both a hydrophilic (water-loving) and hydrophobic (water-hating) end. When placed in water, they spontaneously form circular “vesicles” with phospholipid bilayers because the hydrophobic ends pull together like a drop of oil in water. • BUT, how did the first lipids form?

11. How was early Genetic material replicated? AKA RNA world… • When DNA is replicated by modern cells, several large complex enzymes like DNA polymerase and DNA ligase are required. • Some scientists propose that RNA was the first genetic material because it is simpler and can function as an enzyme. It is also found in viruses, which are not living yet contain genetic material. • Relatively short RNA molecules that can duplicate others have been artificially produced in a controlled lab setting with ideal conditions. But, would it happen in nature?

12. Where did the first Biomolecules and/or living things arise? • In small pools of water on the land like the primordial soup model? (Why would the pools have to be small?) • By ocean vents? By ocean vents extremely hot sulfur-laden water interacts with ocean water, increasing the likelihood of spontaneous chemical formation. • On other planets? Maybe the biomolecules arrived from space. (Seriously, there is a theory called the Interplanetary Dust theory that espouses this idea. Can you identify the problem with this theory?) • On clay? In some experiments, small proteins formed on the surface of clay.

13. HOMOCHIRALITY • Some process in chemical evolution must account for the origin of homochirality, i.e. all building blocks in living organisms having the same "handedness" (amino acids being left-handed, nucleic acid sugars (ribose and deoxyribose) being right-handed. etc). Chiral molecules can be synthesized, but in the absence of a chiral source or a chiral catalyst, they are formed in a 50/50 mixture of both enatiomers. This is called a racemic mixture. • This is a definite challenge to explain, but scientists hypothesize that perhaps the earliest organisms where chiral (using both isomers)but the enzymes that evolved over time selected one enantiomer or another.

14. The inevitable book work • 14.1: 1-5 • 14.2: 1-4 • This is due Friday.