Insects and evolution: some basic questions.

[hedgehog]

I went through the NYS Regents biology course 40 years ago with its implicit teaching of evolution: fish, amphibians, reptiles, birds, mammals, humans studied in sequence. Now, obviously some aspects were incorrect since mammals co-existed with dinosaurs, so mammals branched off before birds developed.

What I don't understand is where do insects come in? Where and when did they branch off? Were they the first air breathers?

[jeff47]

1. It's a tree, not a line.

While many HS biology courses teach evolution as a line, it isn't. There's a lot of independent branches, each evolving in their own niche. You cite an example of this error when you talk about mammals and dinosaurs co-existing (btw, birds were around too).

Another error caused by this teaching is the idea that something like air breathing evolves once, and then all air breathers descend from that ancestor. What actually happens is the same trait can and frequently does evolve independently.

It also doesn't help that cheap and fast DNA sequencing has lead to us completely re-sorting the evolutionary 'tree'.

It is currently thought that insects branched off from crustaceans.

[dipsydoodle]

2. You used the classes of vertebrates

with which insects have no association. Humans are mammals.

edit spellling

[pokerfan]

3. some links

The wiki article on insect phylogeny is huge so here's an eight-page white-paper from Cornell:

http://courses.cit.cornell.edu/ent201/content/diversity.pdf

There were air breathing aquatic animals and it is believed that is where arthropods originated:

Arthropods provide the earliest identifiable fossils of land animals, from about 419 million years ago in the Late Silurian, and terrestrial tracks from about 450 million years ago appear to have been made by arthropods.[49] Arthropods were well pre-adapted to colonize land, because their existing jointed exoskeletons provided protection against desiccation, support against gravity and a means of locomotion that was not dependent on water.[50] Around the same time the aquatic, scorpion-like eurypterids became the largest ever arthropods, some as long as 2.5 metres (8.2 ft).[51]
http://en.wikipedia.org/wiki/Arthropod#Fossil_record

[mike_c]

4. sorry to be late to the party but I have some answers for you....

Our understanding of insect evolution has undergone some radical changes in the last 20 years. First, recall that insects are arthropods, so the first question is "Where did the Arthropoda come from and how did it diversify into it's present form?" This is a VERY important question in modern zoology, because the diversity of arthropods far and away outnumbers the diversity of all other groups of animals. Someone once said, "To a first approximation, all animals on Earth are arthropods."

Arthropods are characterized by having a segmented, multilimbed, multijointed body plan that fundamentally differs from most non-arthropods. It looks primitive because it is-- arthropods evolved during the pre-Cambrian and have simply refined their unique body plan since. They look so alien because they take fundamentally different approaches to solving most of the same biomechanical problems that other invertebrates and vertebrates solve in different ways.

Arthropods diversified in the ocean. The second largest group of arthropods is Crustacea, and current evidence suggests that marine crustaceans invaded brackish and freshwater habitats beginning at the marine-terrestrial interface, and that one group then underwent a profound structural modification (likely a Hox gene mutation) that locked them into the hexapod body plan that characterizes insects today-- three body regions with different specializations, three pairs of walking appendages, (two pairs of wings came later), one pair of antennae, (mostly) no appendages on the abdomen, and so on. That lineage moved onto land and gave rise to some minor relatives of insects, the ectognathous hexapods, and the first group of primitively flightless insects, similar to modern silverfish and firebrats.

Here's a link to a PDF copy of a summary of arthropod evolution I use in my general entomology class: https://dl-web.dropbox.com/get/teaching_110322/zool358/pdf_docs/arthropod_evolution_090630.pdf?w=8bb7f8a6 (hope this works).

And here's one on insect evolution specifically: https://dl-web.dropbox.com/get/teaching_110322/zool358/pdf_docs/insect_evolution_090717.pdf?w=a2b58be9

Hope this helps!

--Mike C.

[eridani]

5. Cool! That's why this group is so greart! n/t

[Yo_Mama]

8. I can't get into your links

403 error, and it wants me to sign in. I'd really like to read the summaries.

[mike_c]

9. this should work-- fixed links....

Sorry about that. I could get in because it's my dropbox acct so I couldn't see that it wasn't working. Try these:

Arthropod evolution: http://dl.dropbox.com/u/23922668/arthropod_evolution_090630.pdf

Insect evolution: http://dl.dropbox.com/u/23922668/insect_evolution_090717.pdf

[Yo_Mama]

10. Thanks - I can get in now.

I really appreciate it.

PS: Reading it. I had no idea that arachnids arose so early, and let's just say that my age is showing - there are huge differences from what I learned lo those many years ago when I was a young sprat.

[mike_c]

13. oh yeah-- animal phylogeny has changed a LOT in recent years....

It's still changing, too. The last three editions of the zoology text I use all have significant phylogenetic differences at the phylum and class levels. The revisions based on molecular data are coming fast and furious these days!

[Yo_Mama]

11. What about the current thinking on sea spiders?

I have been bothered for years with the suspicion that there's a clue to arachnid evolution in the odd characteristics of some sea cucumbers that have organs attached to the respiratory system which secret a sticky mass, which they then eject in long strands to foozle predators. Maybe a common ancester?
http://icb.oxfordjournals.org/content/42/6/1107.full

In some species these apparently contain a toxin.
http://library.thinkquest.org/C007974/2_1scs.htm

[mike_c]

12. I'm a bit confused....

Your subject line refers to sea spiders-- pycnogonids-- but your message text is about sea cucumbers. They're not even remotely related. Sea spiders are arthropods in a small and rather bizarre class, so that makes them protostomes (currently classified in the Ecdysozoa, a super-phylum that nobody talked about when I was in college, LOL). Sea cucumbers, on the other hand, are echinoderms so they're deuterostomes, like vertebrates. Protostomes and deuterostomes likely diverged at least 560 million years ago, but the precambrian fossil record has scant evidence of that, so we really don't know how much earlier they separated. Anyway, it's sufficient to say that arthropods and sea cucumbers are not close relatives. Also, sea spiders are not arachnids despite the common name-- Pycnogonida is a sister taxon within the Chelicerata, however their chelifores are no longer considered homologous with chelicerae, so there is talk of moving them to a unique classification outside the Chelicerata within the arthropods. In any event, they are not closely related to spiders, and likely are even more distant than they've previously been considered.

Interestingly though, that IS about the same time that the arthropods diverged from their parent lineage, which is still pretty hotly debated, about 555 MYA. Echinoderms are presumed to have arisen within the Deuterostomia a bit later, about 530 MYA, although that isn't very clear either, as there are some radiate fossils from much earlier whose affinities are not at all clear.

Not sure whether any of this is relevant to your question though. I don't know much about echinoderms beyond the introductory zoology level-- I'm an entomologist and ecologist, so my phylogenetic interests are pretty firmly rooted inside the Arthropoda, a long way from deuterostomes!

On edit-- oh, I think I get it-- you're thinking of analogies between spider silk and sea cucumber whatyamacallits, um Cuvierian tubules? I don't recall. Anyway, no-- spider silk is a protein, while those sticky strands that sea cucumbers eject are actual tissues, part of the respiratory system.

[Yo_Mama]

15. I was wondering about the timing

I really was wondering what we know or can reasonably infer about the parent lineage of arthropods in particular. I've read that sea spiders are a more recent divergence, and I've read that they might be the remnant of an earlier group that led to arthropods.

In a way it makes sense that the chelicerates and insects are so old, because otherwise I don't know how one would explain grasshoppers, as an example.

But it's also confounding, because if the theories are right, you have this stupendously speedy speciation in less than 200 million years and then not much changes since then for many of the insects. After Pangea breaks up, we pretty much have the modern families? Maybe it was the birds and the mammals eating them that changed things.

[mike_c]

16. THAT is still a big mystery....

I mean, the Onycophora are sister taxa, as are tardigrades, but arthropods diverged from their parent lineage a LONG time ago, and unfortunately the earliest stages of arthropod evolution, at least as hypothesized by most folks, were soft bodied and didn't leave much in the way of fossil evidence. PLUS they were likely much less abundant than the more derived Arthropoda eventually became, because during those early, putative evolutionary stages they lacked most of the features that eventually made arthropods so successful. But in any event, all we have at this point is conjecture. Several centuries worth, actually.

The old view that arthropods and annelids shared a common ancestor is utterly discredited now-- it obviously didn't have much more than conceptual support anyway, since the fossil record is so poor, and the cytochrome-c molecular sequence data has completely dissociated Arthropoda and Annelida into the Ecdysozoa and Lophotrochozoa respectively. No more common ancestor. So where the arthropods came from is STILL one of the big mysteries in zoology.

on edit-- and yeah, diversification among insects happened pretty fast at the order level, so by the time the angiosperms emerged and radiated most of the accompanying insect diversification was at lower taxonomic levels, e.g. genera and families.

[Humanist_Activist]

6. Imagine the world of about half a billion years ago...

The land, what there is of it is completely barren, the atmosphere would suffocate you in seconds, and even the oceans are the wrong color to our eyes, having a greenish tinge from dissolved iron within them. The Earth will appear lifeless, however, it was far from lifeless at this time, stromatolites are forming on the shallow coasts, many single celled cynobacteria and algae are growing in plumes and such floating in the ocean.

This will be important later, but looking deeper in the ocean, you will see what looks like fronds or ferns on the ocean bed, these aren't plants, but rather sessile animals, anchored to the sea floor, filtering food out of the currents. The reason the oceans are greenish in tinge is because it is really low in oxygen, and as a result, these first animals, multi-celluar organisms that can't produce food on their own, but must ingest it, do not possess much energy, and hence stay in one place all their lives. They reproduce in ways similar to corals today, basically throwing eggs and sperm into the currents and allowing them to fertilize into larva that are free swimming, and the first thing they do is seek out a spot to anchor, then stay there for the rest of their lives.

But the ocean is changing, slowly at first, but picking up speed fast, the cynobacteria, algae and other photosynthesizing organisms have been emitting oxygen into the oceans for millions of years, and this oxygen has been absorbed by the iron, which falls to the sea bed as rust, but the dissolved iron has now run out(mostly), and the oxygen is persisting in the water for longer periods of time. Animals, mostly sessile at this point, are beginning to take advantage of this great oxygenation event, it also lead to a mass extinction(many organisms had poor oxygen tolerance at this time, it was their own waste, after all, and many choked on it).

Some species never matured, as it were, but remained free swimming, these animals started actively seeking out food, which increased their survival, and allowed them to avoid danger. They fed on the unicellular life of the oceans and on other animals. The first predators and prey are now existing, soft bodied creatures that mostly resembled worms and other simple animals of today, but they began specializing, and, 200 million years in the future, some of these soft bodied creatures developed hard exteriors, legs and fins, and will come to be known as arthropods. Another branch of the tree evolved in a different way, rather than being tough on the outside, they were becoming tough on the inside, first by developing a rod to protect a nerve cord that ran their length of their bodies, and then eventually that rod developed into a backbone and internal skeleton to protect the nerve cord and vital organs.

They maintained much of the flexibility of their soft bodied ancestors, something the arthropods lost, and became extremely adaptable, eventually evolving a distinct head, a skull to protect one end of the nerve cord that was the brain, etc. The arthropods, in the mean time, began to take advantage of the increased oxygen levels in the air, and developed simple book lungs that allowed some of them, namely scorpions and such, to travel onto land for limited periods of time. They branched out into freshwater streams, and following food sources, namely the newly evolved land plants, deeper onto land, they evolved into insects, spiders, and other land arthropods.

The vertebrates, still in the ocean, would follow millions of years later.

[Dead_Parrot]

7. Others have given more detailed answers, but...

It's time to say hello to TOLweb, the evolutionary equivalent of TVTropes:

http://tolweb.org/Arthropoda/2469

[hedgehog]

14. Thanks folks, for taking the time to point me in the right direction!