Adaptation to a Viscous Snowball Earth Ocean as Path to Complex Multicellularity (2021)

Some of the algae, i.e. the red algae and the green algae, appear to have acquired multicellularity several hundred million years before the animals, i.e. around one billion years ago, if their fossils have been interpreted correctly.

The water viscosity hypothesis would not have been applicable to algae anyway, because their multicellularity has been developed to enable them to stay in a fixed position, attached to the ocean floor, not to move faster, like in the case of animals.

Similarly, the fungi have acquired their multicellularity later than the animals, after more than one hundred million years and their multicellularity has been developed during their adaptation to a terrestrial life, where fungi have lost their mobility due to acquiring a cell wall that prevents drying in air, but also movement.

For the animals, this hypothesis could be true, at least as a partial explanation of the advantages of multicellularity at that time.

In any case, besides the inappropriate mentioning of algae and fungi, the article has also other places with sloppy editing. It should have been revised more carefully.

For instance it says "In eukaryotes, adding cilia or flagella does not increase motility, nor does increasing cell size lead to a major increase in velocity", which is false and the article itself contradicts immediately this sentence by writing "Ciliates are much faster than flagellate eukaryotes".

Indeed, ciliates are much faster than flagellates, because they are bigger and they add a lot of cilia, which results in a major increase in their velocity, as correctly said in the second sentence, in contradiction with the first sentence.

So one way to overcome viscosity is the way of the ciliates, which, instead of associating multiple cells into a bigger body, like animals, have developed a more complex cellular structure, which allows bigger cells with a great number of cilia. There are many kinds of very small animals, with sizes typically under one millimeter, like rotifers and gastrotrichs, which are almost indistinguishable in size, speed and behavior from ciliates.

So up to a certain size threshold the two ways, of ciliates and of animals, are equivalent, but the ciliates have a size limit that is easily overcome by the multicellular animals.

It is very likely that the ancestor of all animals looked very similar to a huge ciliate, i.e. like the ctenophores (comb jellies) still look today. For ctenophores and for several other groups of simple animals the main method of locomotion is by using cilia, not muscles, and this appears to be a primitive characteristic of animals. The replacement of cilia with muscles appears to be a later development, caused by the evolution towards greater sizes, where cilia were no longer efficient.

All the sedentary groups of animals, like sponges and cnidarians, are unlikely to be primitive in this respect, but they have evolved from mobile ancestors. There are many well known cases when some groups of animals have evolved from a mobile life style to a sedentary life style, and then a few of the sedentary have then evolved to be mobile again, but in such cases those that have evolved to be mobile from sedentary, like medusae or salpae, have developed more peculiar ways of locomotion, they have never regained the same method of locomotion of their mobile ancestors. Therefore the evolution of most mobile animals from sponges is far more unlikely than the evolution of sponges from ctenophore-like ciliated animals, which is a kind of evolution that has been seen in a very large number of examples in other animals, for instance in tunicates.

The prevalence of unicellular life on Earth should tell you that this is a malformed question.