Cells to Organisms 3 Eukaryotic cell was a giant leap forward in the early history of life



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Cells to Organisms 3

  • Eukaryotic cells are much more complex, took another 2 billion years to evolve

  • Still carry the “ghosts” of our distant bacterial ancestors in our genes

  • Properties of eukaryotic cells:

  • Usually multicellular, can be unicellular

  • Nucleus is enclosed by a nuclear membrane

  • Cellular organelles enclosed by membranes

  • Genes have introns, “headers” that separate the sections of actual code

  • Have a cytoskeleton - inner framework of actin filaments, microtubules

  • Cytoskeleton was a major step in the evolution of the eukaryotic cell

  • Bacteria, archaea have a rigid cell wall, like plant cells

  • Eukaryotic cells are more flexible, major advantage

  • Eukaryotic cells can

  • Grow much larger, not constrained by cell walls

  • Hold a greater variety of shapes

  • More mobile - actin and microtubules became basis of flagella, muscles etc.

  • Could swallow their prey whole

  • Eukaryotic cells quickly became the dominant form of life on Earth

  • All plants and animals evolved from primitive eukaryotic protozoa and algae

  • The first successful eukaryotes were the single-celled algae and protozoans

  • Kingdom Protista - 65,000-200,000 species, fr. Greek protos = first, ktistos = established - algae, protozoans

  • Taxonomic “grab bag”, primitive organisms only distantly related (polyphyletic)

  • Protists gave rise to all other plants and animals

  • We don’t know how the various groups of protists are related to one another

  • We assume they rose from certain groups of bacteria, but which??

  • Some are autotrophs = algae

  • Some are heterotrophs = protozoa

  • Reproduce either sexually or asexually (by binary fission)

  • Complex life cycles

  • Protists are so different from one another, many may have evolved independently from different groups of bacteria

  • First evolved ~ 1.2 billion years ago

  • As many as 50 phlya recognized

  • Green algae - 7,000 species

  • Ancestral to green plants

  • Use chlorophyll a and b for photosynthesis

  • Cell walls of cellulose and pectin

  • Store food as starch

  • Eukaryotic cells eat by phagocytosis

  • Engulf food in cell membrane

  • Pinch off membrane to form a vacuole

  • Vacuoles store food, water, enzymes, wastes

  • Phagocytosis made it possible for early cells to swallow entire bacteria

  • Opened the pathway by which cells acquired mitochondria and chloroplasts

  • Eukaryotic cells evolved from endosymbiosis (= “shared life within”)

  • Primitive cell devoured another cell that was not digested, became part of the cell that had eaten it

  • Mitochondria, chloroplasts were once independent, free-living organisms!

  • Every cell in our body may be a miniature colonial organism

  • Mitochondria are organelles that specialize in generating energy (ATP - plants and animals)

  • Chloroplasts are the organelles where photosynthesis occurs (plants, protists)

  • How do we know they were once independent organisms??

  • Both have their own set of chromosomes, which are similar to bacterial chromosomes

  • Both have a complete and independent apparatus for protein synthesis

  • Both divide independently of the “parent cell”

  • Many examples of such “internal symbiosis”

  • Euglena - photosynthetic protozoan, may have formed by endosymbiosis, engulfed green algae cell

  • Paramecium bursarium is filled with symbiotic green algae (Chlorella)

  • At least six types of protists seem to have been formed by endosymbiosis

  • The relationship can get very complex

  • Some organisms are endosymbionts within endosymbionts

  • One type of protozoan was formed by eating a red algae, which in its turn was formed by a more ancient protozoan eating a cyanobacteria…

  • Like a set of Russian Matryoshka dolls…

  • Endosymbiosis might explain why archaeans are a mixture of bacterial and eukaryote genes

  • Maybe an ancient archaean was eaten by a bacteria, came to dwell inside

  • Bacterial genes were gradually transferred to the archaean, which became the “nucleus” of a more organized cell

  • Horizontal transfer of genes is common in bacteria (plasmids)

  • Organism later acquired mitochondria and chloroplasts in the same fashion

  • The “ghost’ effect of this type of gene transfer is visible in mitochondria today

  • They have their own genes (RNA) and divide independently

  • But mitochondria can only make ~1% of the proteins they need to survive

  • They rely on the cell nucleus for the rest

  • May be a case of transferring mitochondrial genes to the central cellular control of the nucleus

  • Hartman and Fodorov (2002) think the eukaryotic cell might have formed from a three-way partnership

  • Looked at the genes in a typical eukaryote

  • Took away all the genes found in Archaea and in Bacteria

  • Big step in the early history of life was the evolution of coloniality

  • Coloniality first evolved within eukaryotic cells

  • Multicellular plants and animals evolved from these cells

  • Groups of cells working for the common good

  • Once eukaryotic cells and colonial organisms were commonplace, pace of evolution seems to have increased very rapidly

  • Most of our modern animal body plans evolved in the blink of an eye during the early Cambrian - Cambrian explosion

  • Several colonial forms of green algae, like Volvox

  • Volvox colonies contain 500-60,000 cells

  • Volvox colony has polarity, head and tail end

  • Special reproductive cells at tail end

  • Flagella from surface cells cause colony to spin clockwise as it rolls along

  • Many simple colonial animals

  • Cupulita lives in marine plankton, has sticky thread cells to catch prey, gas-filled cells for buoyancy, reproductive cells…

  • Multicellular bodies are more adaptive in many ways, opens many new potential pathways in evolution

  • Cells cooperate for the common good

  • Not altruism per se, best strategy for survival

  • Higher animals probably began with a Volvox - like ancestor

  • Early stage of all animal development is a tiny hollow ball of cells filled with fluid

  • Blastula has only one layer of cells

  • Ball rolls in on itself to form two layers

  • Third layer forms later between the two

  • Three germ layers form the body plan of all higher animals

  • Simplest multicellular animals today are the sponges

  • Sponges are a loose confederation of specialized cells

  • Volvox shows us another great transition in the early history of life - the evolution of death

  • Volvox daughter colonies form inside parent colony

  • Parent colony must burst to release them

  • Single-celled organisms are immortal

  • Price of multicellularity is death - specialization means certain cells must die so that other cells can live

  • The death of the individual organism is an exchange for a new type of immortality

  • Individuals may die, but their germ cells are passed along (egg and sperm)

  • Now we can summarize the baby steps in the evolution of higher organisms:

  • Formation and concentration of complex organic compounds, prebiotic evolution

  • First organisms evolve from the primordial soup, feed on these abundant organics

  • “Prefab” food dwindles, a few organisms evolve ability to feed themselves (autotrophs)

  • Baby steps in the evolution of life:

  • First metabolic pathways emerge (fermentation)

  • Metabolism based on ADP/ATP appears

  • First photosynthetic organisms evolve

  • Oxygen from early photosynthetic organisms accumulates in the atmosphere

  • Baby steps in the evolution of life:

  • Oxygen atmosphere starts to poison older, anaerobic organisms

  • Some organisms evolve to take advantage of higher oxygen levels

  • Evolve more efficient metabolic pathways using oxygen to “burn” food for energy

  • Baby steps in the evolution of life:

  • Increased efficiency of cellular metabolism leads to eukaryotic cell types

  • Endosymbiosis results in modern eukaryotic cells

  • Multicellular organisms evolve via coloniality among unicellular protists

  • Still a very big gap between

  • Assembling the basic building blocks in Miller-type experiments

  • Forming a complex cell that can reproduce itself

  • Remaining problems include:

  • How proteins took over catalyst role from RNA

  • How special types of RNA evolved (tRNA etc.)

  • How chromosomes formed from genes

  • How cell membranes originally formed

  • How early cells functioned with only few genes

  • The long and complex history of life on Earth may never be completely told

  • But we know enough to chart its tenacious course through eons of geologic time

  • The origin and early evolution of life altered the structure of the Earth itself



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