A good example of how collections of single living cells can transiently associate into a multicellular organism is the Dictyostelium discoideum, a type of slime mould found in ponds. The interactions of single cells of this organism (amoebae) follow a reaction-diffusion process. Millions of these single amoeba cells swim freely in ponds and when the ponds begin to dry up, the amoebae periodically release a chemical substance (cyclic adenosine monophosphate, cAMP) that acts as a messenger to other amoebae. The rhythmic diffusion of cAMP to the receiving cells triggers spatio-temporal patterns of activity leading to their aggregation into a complex multicellular organism1. The process involves oscillatory scroll waves2 and vortex-like patterns spiral movements that follow the equations of the BZ reactions. They form stems slowly growing upwards to a so-called ‘fruiting body’ containing spores that can remain dormant for a long time. When water returns to the pond, the spores become active single cells again and the cycle can then continue.
During the evolution and development of living organisms, the processes that generate new levels of organisation involve assembly of the underlying elements over time. Interactions between living cells that appeared about 2 billion years ago eventually resulted in the appearance of early multicellular organisms 635 million years ago in the Ediacaran period, although these organisms are still hard to classify3.
No living organism has this body plan.
From: https://en.wikipedia.org/wiki/Ediacaran
The Cambrian period, 540-485 million years ago, saw an explosive radiation of animals with new body plans belonging to the evolutionary lines of extant phyla, including the Chordates, the phylum to which humans belong. Recent genetic analyses have shown that comb jellies (ctenophores) are the closest living relatives of these first multicellular animals, but have remained on their own unique evolutionary path4.
from the Burgess Shale, about 508 million years old.
From: https://en.wikipedia.org/wiki/Pikaia
From: https://en.wikipedia.org/wiki/Ctenophora
The fundamental chemical fuel for all living organisms on Earth is glucose (a monosaccharide sugar) synthesised by cyanobacteria, algae, and plants from solar energy by the process of photosynthesis, which therefore feed themselves and are known as ‘autotrophs‘. Some chemotrophic bacteria and autotrophic archaea do not rely on photosynthesis but obtain their energy from inorganic sources. All other living beings must feed on other living beings and are known as ‘heterotrophs‘. Plant eaters are called ‘herbivores‘, animals which eat other animals are called ‘carnivores‘, and animals which eat both plants and other animals are called ‘omnivores‘.
The process by which species appeared, evolved, and diversified has become part of the naturalistic explanation of biological evolution by Charles Darwin, involving genetic variation with random mutation, natural selection, and time. The modern theory of evolution is now well founded in molecular biology and genetics and is no longer doubted.
I will not cover in this essay the process of morphogenesis, ie, how multicellular organisms are generated from single cells.
- This is a form of chemotaxis. ↩︎
- Florian Siegert & Cornelis J. Weijer (1992): Three-dimensional scroll waves organize Dictyostelium slugs. Proceedings of the National Academy of Science, USA 89 6433-6437. Click here to download the PDF version.
Florian Siegert & Cornelis J. Weijer (1995): Spiral and concentric waves organize multicellular Dictyostelium mounds. Current Biology 5(8) 937-943. Click here to download the PDF version. ↩︎ - See: https://en.wikipedia.org/wiki/Ediacaran ↩︎
- Shultz, DT, et al (2023): Ancient gene linkages support ctenophores as sister to other animals. Nature 618: 110-117. ↩︎