1. A 2.1 HL The origin of cells (1).pptx

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1. A 2.1 Origins of cells (HL) “What plausible hypothesis could account for the origin of life?” “What intermediate stages could there have been between non-living matter and the first living cells?”

2. A2.1.1 AHL Conditions on early Earth and the pre-biotic formation of carbon compounds Include the lack of free oxygen and therefore ozone, higher concentrations of carbon dioxide and methane, resulting in higher temperatures and ultraviolet light penetration. The conditions may have caused a variety of carbon compounds to form spontaneously by chemical processes that do not now occur. A2.1.2 AHL Cells as the smallest units of self-sustaining life Discuss the differences between something that is living and something that is non-living. Include reasons that viruses are considered to be non-living. A2.1.3 AHL Challenge of explaining the spontaneous origin of cells Cells are highly complex structures that can currently only be produced by division of pre-existing cells. Students should be aware that catalysis, self-replication of molecules, self-assembly & the emergence of compartmentalization were necessary requirements for the evolution of the first cells. A2.1.4 AHL Evidence for the origin of carbon compounds Evaluate the Miller–Urey experiment. A2.1.5 AHL Spontaneous formation of vesicles by coalescence of fatty acids into spherical bilayers Formation of a membrane-bound compartment is needed to allow internal chemistry to become different from that outside the compartment. A2.1.6 AHL RNA as a presumed first genetic material RNA can be replicated and has some catalytic activity so it may have acted initially as both the genetic material and the enzymes of the earliest cells. Ribozymes in the ribosome are still used to catalyse peptide bond formation during protein synthesis. A2.1.7 AHL Evidence for a last universal common ancestor Include the universal genetic code, several hundred types of genes. Include the likelihood of other forms of life having evolved but becoming extinct due to competition from the last universal common ancestor (LUCA) and descendants of LUCA. A2.1.8 AHL Approaches used to estimate dates of the first living cells and the last universal common ancestor Students should develop an appreciation of the immense length of time over which life has been evolving on Earth. A2.1.9 AHL Evidence for the evolution of the last universal common ancestor in the vicinity of hydrothermal vents Include fossilized evidence of life from ancient seafloor hydrothermal vent precipitates and evidence of conserved sequences from genomic analysis.

3. Conditions on early Earth https://researchoutreach.org/articles/origin-life-conditions-sparked-life-earth/ Sun formed about 4500 million years ago and Earth formed soon afterwards as gravity caused gas and dust to come together. Temperatures were higher and the composition of gases in the atmosphere was very different from now.

4. Conditions on early Earth https://www.britannica.com/topic/evolution-of-the-atmosphere-1703862 Compare the conditions of the Earth’s prebiotic atmosphere with the one today. What are the consequences? Oxygen concentrations were very low because it had reacted with other compounds (e.g. iron) Methane concentrations were high due to volcanic activity and meteorite bombardment Carbon dioxide concentrations were high due to emissions from volcanoes. Temperatures were higher – greenhouse Ozone layer did barely exist because of the lack of O 2 – UV radiation would have been higher

5. https://www.quora.com/Why-have-oxygen-levels-in-the-atmosphere-fluctuated-so-much-over-Earths-history Conditions on early Earth The oxygen content of Earth’s atmosphere has considerably changed over the last 4.5 billion of years. The concentrations was at zero, and has now stabilized at ca. 20%. https://asm.org/Articles/2022/February/The-Great-Oxidation-Event-How-Cyanobacteria-Change Read the article below and answer the questions.

6. Conditions on early Earth

7. It is difficult to define what life is – typically the MRHGREN characteristics of life are used to describe what is required to maintain life, not life itself. A key difference between living and non-living thing is that living things use energy to keep themselves in a highly ordered state. Cells are self-sustaining with all its components, while the individual components on its own would not be considered to be alive. Cells as the smallest units of self-sustaining life

8. Cells as the smallest units of self-sustaining life M etabolism R esponse H omeostasis G rowth R eproduction E xcretion N utrition https://en.wikipedia.org/wiki/Virus#/media/File:Coronavirus._SARS-CoV-2.png MRHGREN Checklist A virus is a very powerful pathogen. Is it alive or not alive? Yes No

9. Throughout the 1700 and 1800’s the idea of abiogenesis and spontaneous generatio n of life (rats from sweaty grain, maggots from meat, etc) persisted – until Louis Pasteur (and others) disproved the idea in 1864. By preventing entry of airborne particles to a nutrient broth , he stopped growth of the culture. Leaving it open allowed microbes to grow. It seems obvious to us but was a huge step forward for cell biology. http://a2.files.biography.com/image/upload/c_fit,cs_srgb,dpr_1.0,h_1200,q_80,w_1200/MTE5NTU2MzE2MzM5NTM3NDE5.jpg Challenge of explaining the spontaneous origin of cells

10. In 1668, Francesco Redi challenged the idea that maggots arose sponatenously from rotting meat. Can you tell how? Challenge of explaining the spontaneous origin of cells

11. Challenge of explaining the spontaneous origin of cells

12. https://youtu.be/AndOnmx9cuM Challenge of explaining the spontaneous origin of cells

13. Challenge of explaining the spontaneous origin of cells Two main theories about how life originally formed in the universe exist: Abiogenesis – life developed independently from non-living or inanimate substances ( spontaneous generation ). Biogenesis – complex life only arises from pre-existing living things through means of reproduction ( non- spontaneous generation ). Proposed by Stanley Miller & Harold Urey. Not scientifically supported (although the Miller & Urey experiment attempted to provide evidence. Mostly based on observations. Proposed by Theodore Schwann & Mathias Schleiden Scientifically proven by experiments (Pasteur, Roedi) Based on practical experiments and material evidence. Does not explain how the first cells arrived on Earth. Could explain how the first cells have arisen on Earth.

14. http://www.reasons.org/Media/Default/ImageCache/559x400-FitWidth/Images/bacteria-1.jpg There was non-living synthesis of simple organic molecules (from primordial inorganic molecules) These simple organic molecules became assembled into more complex polymers These molecules became packaged into membranes with an internal chemistry different from their surroundings (protobionts) Certain polymers formed the capacity to self-replicate (enabling inheritance) How can structures as complex as the cell have arisen by natural means from non-living material? Cells could have arisen from non-living materials Challenge of explaining the spontaneous origin of cells

15. Stanley Miller and Harold Urey`s experiment simulated the atmosphere at the early Earth and managed to create amino acids and carbon compounds needed for life from a mixture of methane, ammonia and hydrogen. In 1953, he (under his University of Chicago mentor, Nobelist Harold C. Urey) performed a famous experiment (the so-called Miller-Urey experiment ) to determine the possible origin of life from inorganic chemicals on the primeval, just-formed Earth. Water was boiled to reflect the high temperatures common to Earth`s original conditions. The vapor was mixed with gases (H 2 , CH 4 , NH 3 ) to create a reducing atmosphere (no O 2 ). Electrical discharge (“lightning”) as energy source lead to reactions. After cooling, a week later, the mixture was analyzed and traces of organic molecules were found. Evidence for the origin of carbon compounds

16. https://youtu.be/aHxxNqEN_Gk Evidence for the origin of carbon compounds

17. If phospholipids were among the first carbon compounds, they would naturally assemble into bilayers. These bilayers readily form vesicles, resembling the plasma membrane of a small cell. This allows compartmentalization. Spontaneous formation of vesicles

18. DNA replication requires the action of enzymes . However, for enzymes to be made, genes are needed. The solution to this conundrum may have been an earlier phase in evolution when RNA was the genetic material . RNA can store information in the same way as DNA but is both self-replicating and can act as an enzyme. http://discoverevolution.com/files/2012/10/ancestry-dna-evidence.jpg RNA as a presumed first genetic material The polymers formed the capacity to self-replicate

19. RNA as a presumed first genetic material https://en.wikipedia.org/wiki/Ribozyme#/media/File:Ribozyme.jpg This theory is further supported because of RNAs ability to act as a catalyst because it can form complex 3D structures of polypeptides. For a long time it was believed that only enzymes are complex enough to act as catalysts in reactions. RNA can act as a catalyst by cutting introns from messenger RNA, and by catalyzing the change of a precursor tRNA into active tRNA.

20. Evidence for a last universal common ancestor https://youtu.be/de1hiS_XjWg

21. Evidence for a last universal common ancestor All living organisms store information in the genetic code, and because they all use the same 4 letter code which translates into codons specific to the same amino acids it is said to be universal. Because of this shared feature, all organism can be traced back to a common ancestor (Last Common Universal Ancestor, LUCA). https://www.newscientist.com/article/2098564-universal-ancestor-of-all-life-on-earth-was-only-half-alive/

22. Evidence for a last universal common ancestor https://www.quora.com/Reconstructing-the-Last-Universal-Common-Ancestor-LUCA-is-challenging-because-horizontal-gene-transfer-appears-to-have-been-common-among-early-life-forms Is a different genetic code possible? Convergent evolution of same structures evolving is a possibility. This does not to seem to be the case for the genetic code. It is likeley that other forms of live evolved – but they may have become extinct due to competition from LUCA or species that evolved from LUCA in the process of natural selection.

23. E stimating dates of the first living cells and LUCA https://www.bbc.com/travel/article/20210117-stromatolites-the-earths-oldest-living-lifeforms Research has shown that in Western Australia 3.5 billion-year-old stromatolites (rock mounds built up by cyanobacteria) established the oxygen content of the Earth’s atmosphere. The rocks were isotope dated and it showed remains of life. No rocks older than 3.5-4 bio years exist (destruction by tectonic movement, heat or pressure), so only isotope ratios ( 13 C/ 12 C) can be used for the dating of the first living cells using the hardest fragments that can exist.

24. E stimating dates of the first living cells and LUCA https://mikegolby.com/humanorigins/was-life-on-earth-inevitable/

25. A possible origin of complex carbon compounds is around deep-sea vents. Hydrothermal vents (black smokers) are found at fissures in the oceanic crust, gushing hot water which often carries reduced inorganic chemicals such as iron sulphide, hydrogen, methane or ammonia. These chemicals are used as sources of energy for the assembly of simple carbon compounds into polymers. http://deepseacreatures.org/images/stories/deep-sea-vent.jpg Evidence for the evolution of LUCA near hydrothermal vents

26. https://www.nytimes.com/2016/07/26/science/last-universal-ancestor.html Evidence for the evolution of LUCA near hydrothermal vents https://www.nytimes.com/2016/07/26/science/last-universal-ancestor.html Researchers have identified 355 genes coding for protein families that occur widely among bacteria and archea bacteria – the organisms which are believed to have originated first in the history of life. These genes were inherited from an early common ancestor – LUCA. Many of the genes are needed for anaerobic metabolism and carbon dioxide fixation – conditions preferably found around hydrothermal vents. Read more about this here !