It's a mystery. But - bacteria got on fine without multi-cellular life for nearly 4 billion years. Only had multi-cellular life for a little over half a billion years since the Cambrian explosion, with some small simpler animals for a few hundred million years before then. Animals - origins
Nobody really knows what caused the Cambrian explosion, though lots of theories. Cambrian explosion
Perhaps we might get a better idea about this if we find independently evolved life either in our solar system or around other stars.
Perhaps life on Earth was unusually slow at developing multi-cellular animals for some reason? On the other hand we might be the fastest off the block, even the first in our galaxy to have multi-cellular life. No way to know.
Before then - did have microbial mats and stromatolites, so those are comparatively easy to form. Once you have microbes, it's not too surprising that they might come to live together in communities with various microbes forming somewhat symbiotic relationships with others in the mat.
Also did get very large single celled lifeforms.
The Eukaryotes, microbes with a nucleus (separate "compartment" for the genetic material) - they evolved perhaps 1.6 to 2 billion years ago - but could be older than that.
Eukaryote
So again - that might be quite a hard evolutionary step as well.
On Earth it seems to be a necessary precursor for multi-cellular life as only the Eukaryotes evolved into fully fledged multi-cellular lifeforms, though some prokaryotes have multicellular stages in their life cycles.
Here is an example of a multicellular stage for a cyanobacteria, showing that a cell doesn't have to have a nucleus to be multi-cellular, at least in a simple form:
However, evolution doesn't seem to have taken this too far and the more complex forms of multi-cellular life are all eukaryote based.
You can read some of the hypotheses for origins of multicellularity here:
Multicellular organism - hypotheses for origin
Nobody really knows what caused the Cambrian explosion, though lots of theories. Cambrian explosion
Perhaps we might get a better idea about this if we find independently evolved life either in our solar system or around other stars.
Perhaps life on Earth was unusually slow at developing multi-cellular animals for some reason? On the other hand we might be the fastest off the block, even the first in our galaxy to have multi-cellular life. No way to know.
Before then - did have microbial mats and stromatolites, so those are comparatively easy to form. Once you have microbes, it's not too surprising that they might come to live together in communities with various microbes forming somewhat symbiotic relationships with others in the mat.
Also did get very large single celled lifeforms.
The Eukaryotes, microbes with a nucleus (separate "compartment" for the genetic material) - they evolved perhaps 1.6 to 2 billion years ago - but could be older than that.
Eukaryote
So again - that might be quite a hard evolutionary step as well.
On Earth it seems to be a necessary precursor for multi-cellular life as only the Eukaryotes evolved into fully fledged multi-cellular lifeforms, though some prokaryotes have multicellular stages in their life cycles.
Here is an example of a multicellular stage for a cyanobacteria, showing that a cell doesn't have to have a nucleus to be multi-cellular, at least in a simple form:
The cells are contained in a tube giving it a kind of a structure - and the large cells shown with arrows specialize in nitrogen fixing, making it a single organism with differentiated cells for different tasks - although rather simple compared with most multi-cellular eukaryote based lifeforms.
See Bacteria with bodies – multicellular prokaryotes | Lab Rat, Scientific American Blog Network
However, evolution doesn't seem to have taken this too far and the more complex forms of multi-cellular life are all eukaryote based.
You can read some of the hypotheses for origins of multicellularity here:
Multicellular organism - hypotheses for origin
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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