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There have been many changes in life overits long tenure on the earth. Butundoubtedly one of the most remarkablechanges was the development. Ofmulticellularity.What is a multicellular organism? Well amulticellular organism is generally onemade up of cells that have irreversiblydifferentiated or altered to carry outspecialized functions. For example in uswe might have cells that are growing intoliver cells, some cells will become skincells And cells that do other functions.These different diversity of specializedcells come together to make the wholehuman organism. This is generally whatmulticellular organisms are. At some pointin the history of life on Earth,unicellular organisms or microorganismstransition to become multicellular. Howdid this happen, and when did it happen?For the first 2.5 billion to 3 billionyears of life on Earth, there were only

microorganisms. This evidence is preservedin the early rock record in controversialmicro-fossils and chemical signatures.These remnants of early microorganismsbecome better preserved as the rock recordimproves through time. And then between50085 million years ago and 542 millionyears ago, fossils begin to appear in therock record that suggests early multicellular organisms. What is the earliestevidence for this multi cellularorganisms? Well some of the 1st records ofmulti cellular creatures. I found an

Ediacaran Fauna. These are fossilspreserved in rocks between 585 and 542million years ago. They're very enigmaticcreatures, strange, tubular, a frontshaped organisms which lived during thisperiod. And many people think that thesewere experiments in early body plans.Experiments that would eventually giverise to the body plans of multicellularorganisms That we're familiar with today.These were first discovered in Australia.And this is an artist impression of thesecreatures and what they may have looked

like those many millions of years ago. Howdid multicellularity arise? Well, at somepoint, single ce ll microorganisms.Made the transition to multicellularorganisms. We don't quite yet know howthat happened. It's one of the greatquestions in astrobiology, to fathom that,how multicellularity evolved. There aretwo possible explanations. Tonations.One is an internal change in cells, and

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one is possible environmental change. theinternal change ay have occurred by somealteration in the genetics. At some pointwhen cells were dividing, they divided insuch a way as to differentiate some changein the genetic Code caused cells to changetheir functions but remain incommunication with one another such that amass of previously almost identical singlecelled micro-organisms became a mass ofdifferentiated cells, an earlymulticellular organism. But one wouldstill have to question why that happened.What triggered this geneticdifferentiation within cells, thisemergence of multicellular features.Another explanation is the rise of oxygenin the earth's atmosphere. Oxygen makespossible aerobic respiration wherebyorganic material is essential burned inoxygen to produce energy. Aerobicrespiration is a very good way ofgenerating large quantities of energy andit allows the. Things like running and

jumping, and even running a brain. Soaerobic respiration may have been key tomulticellular organisms. The rise ofoxygen in the earth's atmosphere tosimilar levels to today may have allowedlarge, complex organisms to evolve anddevelop. Once it did evolve, why did itremain around? Why was it selected for?Well, as Charles Darwin said, it is notthe strongest of the species thatsurvives, nor the most intelligent thatsurvives. It is the one that is mostadaptable to change. Multicellularity may

have been advantageous in several ways.For example, increased size would haveallowed organisms to escape predation andcompetition from other multi-cellularorganisms. So once it did emerge, itpersisted because it was necessary fororganisms to escape already existing.Multi-celluar organisms.Multi-cellularity may also have allowedorganisms to exploit new ecologicalniches. Regions where there were resourcesor food supplies perhaps not beingefficiently used by other types of life,

allowing life to expand into new physicaland ecological territories. Cellularspecialization is also quite energeticallyefficient. the cells specialize in oneparticular function in an organism. Theycan do that function very efficiently andvery well, and thereby direct their energyto that particular function.Multicellularity may also have allowed forphysical protection from the external

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environment. Protection from otherpredators, but also protection fromphysical extremes. There were alsocomputational advantages. Multicellularorganisms were able to develop much morecomplex behavioral characteristics, whichwould have allowed them to have competedmore effectively. To have hunted out newterritories and new food supplies And newresources more efficiently. We have to becareful in interpreting these changes asnecessary for organisms to compete. Weknow that many microorganisms areenergetically very efficient. We also knowthat they can survive in extremeenvironments very well. And also competewith other microorganisms. So the reasonsthat multicellularity both evolved andpersisted in the environment are stillcontroversial. And one of the questions tobe addressed by astrobiology. One of themore remarkable transitions in the fossilrecord occurs. 542 million years ago, whenwe start to see fossils appearing in the

rock record that represent a hugediversity of multi-cellular organisms.This period in the rock record is calledthe Cambrian Explosion. One of the reasonswhy the diversity of multicellularorganisms may greatly increase in rockrecord of this time is thought to bebecause organisms began to evolve theability to precipitate minerals. Used forskeletons and shells. And once theseskeletons and shells became preserved, webegan to see a greater diversity of life.In th e rock record, before that

multicellular organisms were in generalsoft body and those soft bodied organismsare not well preserved in the rock record.So the Cambrian explosion, this increasein the diversity of multicellularorganisms in the rock record. Maybe anartifact of preservation, but neverthelessit shows us that this is a period of Earthhistory where the diversity ofmulticellular organisms Including theirability to be preserved, was greatlyincreased. I have given you, perhaps, someidea that life on Earth is rather

continuous. We've gone from unicellularmicroorganisms to multicellular life,right the way through to the present day.But it's important to remember thatmulticellular organisms, as probably withmicroorganisms, have been subject to massextinctions in their part. Since the riseof multi-cellular life, we recognize fivebig mass extinctions in the rock record.they have different causes. And the causes

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of these mass extinctions are verycontroversial, another area which is veryactive field investigation inastrobiology. For example, the cretaceouspaleogene extinction 65 million years agois thought to have been caused by anasteroid or comment impact. Other massextinctions have probably been caused bychanges in the environment. Other thanasteroid and comet impacts, for examplefrom volcanic eruptions. All of thesecauses are a matter of debate. But aftereach extinction, bio-diveristy on theEarth has always recovered or increased,showing how resilient life on earth is tothese Large environmental changes that arecapable of causing extinctions.Extinctions may not always be bad for alllife, because of course extinctions createspaces for new organisms that survive toflourish and emerge into empty habitat.For example, the destruction of thedinosaurs 65 million years ago allowedmammals to dominate the surface of the

earth, eventually leading to human beings.So extinctions are a double edged sword.They result in the destruction Of largeswaths of diver sity of life on earth butthey also open up new opportunities forlife, to expand into those empty nichesand habitats. So what have we learned inthis lecture? Hopefully what we'velearned, is that unicellular organisms.Prevailed for about the first 3 billionyears of life's history. Fossils fromabout 585 to 542 million years ago seemedto comprise the first evidence for

multi-cellular life at least experimentsin early body plants. Multi-cellularityarose maybe as a result of interactionsbetween the cells, genetic changes withinorganisms themselves. Or changes inatmospheric conditions, such as a rise ofatmospheric oxygen. The advantages ofmulticellularity may have included anincrease in size and specialization,physical protection from the environment.And even the conditions for thedevelopment of more complex behavior,which would have enhanced their ability to

compete in the environment. The CambrianExplosion, as recorded in the rock record,shows a great diversification of life,including the appearance of hard-bodiedorganisms. In the rock record. Andfinally, mass extinctions on Earth havebeen followed by periods of biologicalrecovery and diversification. Butextinctions have very much been a fact oflife for multicellular life since it first

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arose.