Passing through an embryonic stage is a/the(?) sine qua non of being an animal?

Factual Questions
1

Suggested by current thread First organism large enough to be seen by human eye (if there were humans then), in which Colibri cites “the first macroscopic fossils of single organisms [which] seem to date from about 2.1 billion years ago.”

In 1998 Mark McMenamin claimed Ediacarans did not possess an embryonic stage, and thus could not be animals…

I will never come near to grasping Linnean studies, but I never knew the concise one-liner, basic fact of this division of the kingdoms.

Could someone (paging Colibri of course), help me at the simplest level where/how that statement fits in at this extraordinary high level of basic biology which I feel honor-bound to sort of know. :slight_smile:

2

I’ve never noticed this claim before, though I am somewhat familiar with some of McMenamin’s work, including reading his book The Garden of Edicara. I don’t know the basis for his claim that edicarans don’t have embryonic stages. Plants also have embryonic stages, so you can’t say that everything that has an embryo is an animal; you can say only that every animal developed from an embryo. And that comes from an arbitrary cut-off point in defining what is an animal. Discovering an embryo for an edicaran would not nail down them as being animals, but if you (somehow) proved the negative that they don’t have embryos would prove that they aren’t. (Unless you chose to redefine “animals” to include them–think of it this way–mammals don’t lay eggs, except if you want to call a platypus a mammal, you have to expand the definition to include the possibility of egg-laying mammals.)

We don’t know what lots of the edicarans are, even to the kingdom level. Some say that some of them are animals, some say that they are fungi, some say that they are a completely different line of metazoans that died off completely, some say that they are gigantic unicellular organism.

Also, just because a scientist does some good work doesn’t mean that other ideas of theirs can’t be wacky. This particular scientist believes that there were ancient giant squid creating artwork with ichtyosaur bones.

3

Actually, fossil embryos (or traces interpreted as representing them) are now known from the Ediacarian period. This discovery appears to post-date McMenamin’s statement.

Animals typically develop through a blastula stage, a hollow ball of cells. There may be some very simple animals that don’t have embryos like Placozoa, but these may be descended from ancestors that had them.

Plants (Plantae, multicellular plants) also have embryos, but they are different in structure from those of animals. Fungi do not develop from embryos.

4

There are animals that can reproduce without an embryonic stage; two that come to mind are hydra and asexual planaria. The hydra reproduce by budding, and the planaria by splitting in two and regenerating the missing bits. Some sea stars also split off an arm which then grows into a new individual.