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How Did Single Cells Become the First Animals?

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How did life get from single cells to animal bodies? This 5-minute explainer follows the order of steps, from choanoflagellates, our closest living relatives, through colonies and the genetic toolkit for cell cooperation to the first animals and the Ediacaran fossils. Dates are given as ranges, and the debated parts (benefits of colonies, shape of the first animal, sponges or comb jellies first, sterane biomarkers, Charnia) are marked on screen. Pictures are AI illustrations, not photographs; this is a draft to be fact-checked. 0:00 One cell, thirty trillion cells 0:15 The order of steps 0:29 Bodies evolved many times 0:45 Our closest living relatives 1:18 Colonies by division 1:50 Volvox: a separate route 2:07 The toolkit older than animals 2:24 The first animal: shape and first branch 2:56 Sponges and Trichoplax 3:29 Molecular clocks and chemical fossils 3:47 The Ediacaran fossils 4:40 Take-away and open questions Sources: Brunet & King 2017; King et al. 2008; Carr et al. 2008; Fairclough et al. 2010; Alegado et al. 2012; Herron et al. 2009; dos Reis et al. 2015; Love et al. 2009; Nettersheim et al. 2019; Narbonne 2005; Bobrovskiy et al. 2018; Evans et al. 2020. Full list in references.md. Made with Eureka, a Nebulytic product. Illustrations are AI-generated and schematic; check the facts in the sources before relying on them. Made with Eureka, a Nebulytic product.

Chapters

  • 0:00One cell, thirty trillion cells (hook)
  • 0:15The order of steps
  • 0:29Bodies evolved many times
  • 0:45Our closest living relatives: choanoflagellates
  • 1:18Colonies by division, and why they might help (debated)
  • 1:50Volvox: a separate route
  • 2:07The toolkit older than animals
  • 2:24What did the first animal look like? Sponges or comb jellies first? (debated)
  • 2:56Living simple animals: sponges and Trichoplax
  • 3:29Dates from molecular clocks and chemical fossils
  • 3:47The Ediacaran: Charnia, Dickinsonia, Kimberella, Cloudina
  • 4:40The order of steps and what is still open

Sources

  1. Sources relied on for dates, numbers and claims (title, author or body, year). Bibliographic details were written from memory of the literature and are all "to be verified by author" before publishing (volume and page numbers in particular); no reference was invented, and items marked (?) are the ones the author should check first.
  2. The origin of animal multicellularity and cell differentiation. Brunet T, King N. Developmental Cell, 2017.
  3. The genomic and cellular foundations of animal origins. Richter DJ, King N. Annual Review of Genetics, 2013.
  4. The origin of Metazoa: a unicellular perspective. Sebé-Pedrós A, Degnan BM, Ruiz-Trillo I. Nature Reviews Genetics, 2017.
  5. The evolution of multicellularity: a minor major transition? Grosberg RK, Strathmann RR. Annual Review of Ecology, Evolution, and Systematics, 2007.
  6. The multiple origins of complex multicellularity. Knoll AH. Annual Review of Earth and Planetary Sciences, 2011.
  7. Life on a Young Planet: The First Three Billion Years of Evolution on Earth. Knoll AH. Princeton University Press, 2003.
  8. The Choanoflagellates: Evolution, Biology and Ecology. Leadbeater BSC. Cambridge University Press, 2015.
  9. Choanoflagellates. King N. Current Biology (Primer), 2005.
  10. Volvox: Molecular-Genetic Origins of Multicellularity and Cellular Differentiation. Kirk DL. Cambridge University Press, 1998.
  11. Six major steps in animal evolution: are we derived sponge larvae? Nielsen C. Evolution & Development, 2008.
  12. Choanoflagellates, choanocytes, and animal multicellularity. Maldonado M. Invertebrate Biology, 2004.
  13. Revisions to the classification, nomenclature, and diversity of eukaryotes. Adl SM et al. Journal of Eukaryotic Microbiology, 2019.
  14. Invertebrates (zoology textbook, sponges). Brusca RC, Brusca GJ. Sinauer Associates (current edition).
  15. Developmental Biology (embryology textbook). Gilbert SF. Sinauer Associates (current edition).
  16. The Rise of Animals: Evolution and Diversification of the Kingdom Animalia. Fedonkin MA, Gehling JG, Grey K, Narbonne GM, Vickers-Rich P. Johns Hopkins University Press, 2007. (?)
  17. International Chronostratigraphic Chart (Cryogenian, Ediacaran). International Commission on Stratigraphy (ICS), current version.
  18. Revised estimates for the number of human and bacteria cells in the body. Sender R, Fuchs S, Milo R. PLoS Biology, 2016.
  19. Bangiomorpha pubescens n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes. Butterfield NJ. Paleobiology, 2000.
  20. Precise age of Bangiomorpha pubescens dates the origin of eukaryotic photosynthesis. Gibson TM et al. Geology, 2018.
  21. Molecular phylogeny of choanoflagellates, the sister group to Metazoa. Carr M, Leadbeater BSC, Hassan R, Nelson M, Baldauf SL. PNAS, 2008.
  22. The genome of the choanoflagellate Monosiga brevicollis and the origin of metazoans. King N et al. Nature, 2008.
  23. Premetazoan genome evolution and the regulation of cell differentiation in the choanoflagellate Salpingoeca rosetta. Fairclough SR et al. Genome Biology, 2013.
  24. Phylogenomics reveals convergent evolution of lifestyles in close relatives of animals and fungi. Torruella G et al. Current Biology, 2015.
  25. Multicellular development in a choanoflagellate. Fairclough SR, Dayel MJ, King N. Current Biology, 2010.
  26. Cell differentiation and morphogenesis in the colony-forming choanoflagellate Salpingoeca rosetta. Dayel MJ et al. Developmental Biology, 2011.
  27. A bacterial sulfonolipid triggers multicellular organization in choanoflagellates. Alegado RA et al. eLife, 2012.
  28. Bacterial lipids activate, synergize, and inhibit a developmental switch in choanoflagellates. Woznica A et al. PNAS, 2016.
  29. Cooperatively generated stresslet flows supply fresh fluid to multicellular choanoflagellate colonies. Roper M, Dayel MJ, Pepper RE, Koehl MAR. Physical Review Letters, 2013.
  30. Phagotrophy by a flagellate selects for colonial prey: a possible origin of multicellularity. Boraas ME, Seale DB, Boxhorn JE. Evolutionary Ecology, 1998. (?)
#evolutionofmulticellularity#firstanimals#choanoflagellates#originofanimals#Ediacaranbiota#Dickinsonia#Charnia#sponges#combjellies#molecularclock#Cryogenian#scienceexplainer

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