Deep homology in plant and animal nervous systems?

#1


Interesting, new-ish (from september) finding: Plants communicate distress using the amino acid glutamate through a long-distance defense signalling mechanism. The researchers compare it to the animal nervous system, which also employs glutamate as a neurotransmitter.

An open question is whether the glutamate receptors employed by plants are related to the glutamate receptors used in animals, or whether they evolved independently. From my perspective of Front-Loaded Evolution, I would expect the receptors to be related, dating back to the unicellular last common ancestor of plants and animals.

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(John Harshman) #2

This is your first mention of such a perspective. Some day you should actually lay out your hypothesis, and you should do that before you try to tell us what it predicts.

#3

Front-Loaded Evolution is the conjecture that the first life on Earth was designed with its future evolution in mind, such that some features of early life channelled its future evolution. I’ve written more about it on my blog, most recently here.

(John Harshman) #4

Your first problem, and it’s only the first, is that eukaryotes are not the first life by a couple billion years.

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#5

Indulge me: How old are the oldest bacteria? How old are the oldest eukaryotes?

(John Harshman) #6

I won’t indulge you. You clearly have that information already. What game is this?

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#7

Identifying life at the cellular level in the fossil record is a tricky business, and your dating of the first life depends on how high you set the bar for what you’ll consider sufficient evidence. I’m curious where you set the bar.

(John Harshman) #8

You first. When do you think the first life is? When do you think the first eukaryote life is? (I mean based on the fossil record.)

#10

You’re the one telling me that bacteria proceeded eukaryotes “by a couple billion years”. Where does this number come from? What are the end points?

(John Harshman) #11

I was going with rough estimates of 3.5 billion years for prokaryotes and a generous 1.5 billion for eukaryotes. As you doubtless know, there are various candidates for early eukaryote fossils, some of them dubious. Now you.

#12

I think the eukaryote-sized microfossils from 3.2 byo give a good indication as to the minimum age of eukaryotes.

https://www.nature.com/articles/nature08793

(John Harshman) #13

As you presumably know, the eukaryote nature of those fossils is considered dubious.

(Blogging Graduate Student) #14

Here’s a figure from a 2014 review of eukaryotic origins, Knoll 2014:

It’s notable that molecular clock estimates of the origin of crown eukaryotes also rarely stray past 2 Gya, and are usually more around 1.5 Gya (Eme et al. 2014).

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#15

Wouldn’t this also be a prediction if front-loading was not present? How do your predictions differ from evolutionary processes that lack front-loading?

For example, how do you predict which mutations will occur and which will not? If you looked at the genome of the E. coli used in Lenski’s long-term experimental evolution project, would you have been able to predict which mutations would have occurred beforehand?

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(S. Joshua Swamidass) #16

Is this homology or convergence?

(Ashwin S) #17

Cut him some slack. His idea is nowhere near as crazy as meteorites carrying viruses that helped in each major evolutionary step… until it reaches the pinnacle - the octopus…
And that was a peer reviewed published paper.
IMO @Krauze is not going far enough…

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(Robert Byers) #18

This is interesting for YEC creationism. If god created all biology on a common blueprint then one would predict plants and creatures would have a nervous system/immune system that was very alike. only different as it needed to be.
yet if evolutionism is true then any likeness must be from a common descent original organization of a nervous system/immune system (unless convergent evolution is once more brought to the rescue) and so there has been very little evolution since the animal/plant union days. surely unlikely!
As better research is done the prediction from creationism is that biology will constantly be shown to be from a single blueprint. Homology will rule. there will not be the crazy random results from chance mutations/selection.
Gentlemen lay your bets!

#19

No, that’s not my impression. My impression is that there’s widespread agreement that the fossils are of biological origin, although there’s uncertainty whether they can be classified as eukaryotes.

As I’ve said, identifying microscopic fossils is a tricky business, and as I see my views as little more than a hunch to be investigated, I’m fine with some fuziness around the edges.

Of course, those molecular clock estimates are of the age of the last eukaryotic common ancestor (LECA), not the first eukaryote.

I don’t know, what does conventiontal evolutionary biology predict regarding whether glutamate receptors evolved independently in plants and animals?

Front-loading isn’t dependent on being able to predict which mutations will occur and which will not. Front-loading rests on a recognition of what we could call evolutionary bias.

To understand evolutionary bias, let’s consider eusociality, in which sterile individuals work to further the reproductive success of others. Eusociality has evolved at least 8 times in the insect order Hymenoptera (Søvik et al., 2005), but only once in the mammalian order Rodentia (the naked mole rat).

William D. Hamilton proposed that eusociality evolved in Hymenoptera through kin selection due to the curious genetic sex determination trait of haplodiploidy, in which haploid males are closer related to their diploid sisters than to their brothers. Whether or not Hamilton’s theory proves to be ultimately correct, it illustrates the principle that genetic, developmental or organismal characteristics of an organism can bias evolution by making some parts of the fitness landscape easier to reach than others.

Mind you, the above is perfectly compatible with an ateleological view of life. I’m not suggesting that eusociality is front-loaded. Where it gets really interesting is when we apply the principle of evolutionary bias to things like the evolution of multicellularity. More on this to come in future posts.

Good question. Based on the current evidence, I would consider it convergence. The common ancestor of plants and animals is not believed to have had a nervous system, making it convergent. The researchers have only found that glutamate is used as a signalling mechanism; they haven’t looked at the glutamate receptors themselves. If the receptors are found to be related to animal glutamate receptors (as I suspect them to be), I think it would be appropriate to use the term “deep homology”, as in the deep homology between features like eyes and legs in D. melanogaster and H. sapiens, which are regulated by related homeodomain genes.

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(Blogging Graduate Student) #20

That’s exactly what @John_Harshman said… it’s quite an important point to consider when talking about the origin of eukaryotes.

(John Harshman) #21

As I said.

Are you positing a ghost lineage of stem eukaryotes for billions of years?