How Birds Evolved From Dinosaurs (2015) | Hacker News Reader

How Birds Evolved From Dinosaurs (2015)

quantamagazine.org

Just found this ...

https://en.wikipedia.org/wiki/Hoatzin

> notable for having chicks that have claws on two of their wing digits.

> In 2015, genetic research[5] indicated that the hoatzin is the last surviving member of a bird line that branched off in its own direction 64 million years ago, shortly after the extinction event that killed the non-avian dinosaurs.[6]

Given how much that looks like a bird, that strongly suggests we would also recognize avian dinosaurs as birds, right?

It hasn't remained the same for all that time.

For any feature this bird and other birds have in common to have come later, they would have to have evolved twice. This could be the case for some features, but is highly unlikely to be responsible for large part of the similarities.

My understanding is that non avian dinosaurs are the birds.

You may accidentally have an extra word in, "My understanding is that __non__ avian dinosaurs are the birds."

avian = related to birds

Doh! Yes the _non_ is extra.

> genetic research

How did they get the genetic code of the specific dinosaur branch? I recall a previous discussion here that we have very little genetic source code from dinosaurs because it's degraded in the fossils over time

Any genetic code analysis is going to be from creatures that are alive today, or at most were alive a few thousand years ago. When you're looking at the fossil record, you're basing families of species on the age of the fossils and on morphology -- the shape of bones and teeth, etc.

Aside from birds, the closest living relatives of dinosaurs are crocodilians. Any DNA that birds and crocodiles have in common almost was certainly shared with dinosaurs. Scientists use this genetic bracketing to get a sense of what the DNA of ancient species might have looked like.

In the long run mutations occur at a fairly predictable rate, so geneticists can use statistical analysis to estimate how long ago two different species or families of species may have diverged.

Since the Hoatzin is a living species, geneticists can compare its DNA to the DNA of other living birds. Based on this, they can estimate how long ago Hoatzins diverged from other living birds, and they can then go to paleontologists to compare their estimate against the fossil record. Any DNA that Hoatzins share with crocodiles, but not with other living birds may indicate DNA that birds evolved after their divergence from other dinosaurs, so this offers a glimpse into what dinosaur DNA looked like.

The article is actually really interesting and has a lot more up-to-date information than the title would suggest. For example, one new idea I gleaned from this is that the evolution towards smaller body size happened long before the extinction event 65 million years ago. Miniaturization stretches back at least 200 million years. Other neat fact: early birds looked very much like infantile or even embryonic raptors, suggesting that instead of evolving a whole new set of traits, small modifications to embryonic development were more likely. It seems this idea is also picking up in other areas of studying evolution as well.

> early birds looked very much like infantile or even embryonic raptors

Sounds like neoteny, the retention of juvenile characteristics in adulthood. See: domesticated foxes, dogs (esp toy breeds), people. Seems like a particularly easy evolutionary path, perhaps take a step back, then forward.

> New research suggests that bird ancestors shrank fast, indicating that the diminutive size was an important and advantageous trait, quite possibly an essential component in bird evolution.

This is making the common mistake regarding evolution. Evolution doesn't select for advantageous things, but against disadvantageous things. It's not survival of the fittest, but death of the inadequate.

It seems nitpicky, but it's very important for diversity. If getting larger or smaller doesn't drastically hinder your survivability, then one ends up with a large range of sizes.

Do evolutionary biologists generally agree with what you are saying? Is it what I would be taught if I took a course in evolutionary biology?

Birds did not evolve from dinosaurs, they are dinosaurs.

Just read "Some Assembly Required: Decoding Four Billion Years of Life, from Ancient Fossils to DNA" by Neil Shubin. The pathway how a current species became what it is is quite surprising. Most features you see today came about as copies of other features that were repurposed over time, like feathers did not evolve to support flight, but for other reasons, and then lead to flight later. This idea of repurposing started with a Darwin comment in his last update to The Origin Of Species.

I hear this a lot, but I don't understand in what capacity it is true. Why can we say "Birds did not evolve from dinosaurs they are dinosaurs" but not "Humans did not evolve from fish they are fish"?

Humans can be classified as lobe-finned fish, so it wouldn't be wrong to say that humans are fish.

My question was more along the lines of "is it wrong to say 'Humans didn't evolve from fish because humans are fish'"? This is by all appearances the same argument the GP and others make when they say "Birds didn't evolve from dinosaurs; they are dinosaurs".

Birds are part of what’s called the Dinosauria clade. Clades are monophyletic meaning they encompass all the descendants of a common ancestor and all share common traits.

Humans and fish on the other hand can’t be grouped into a clade. That’s because it you trace back the common ancestor you’ll find other monophyletic groups descending from those ancestors.

Think of a clade as the end of a branch and all its leaves... whereas if you go far enough down you’ll find other branches that shoot off in other directions far off, even though they’re in the same main trunk.

https://en.m.wikipedia.org/wiki/Clade

This seems like a rephrasing of the claim "birds are dinosaurs". "Birds are dinosaurs because birds are dinosaurs". Maybe the answer is "it's arbitrary; scientists decided that birds belong in the group 'dinosaurs' even though 'dinosaurs' and 'birds' are clearly distinct groups to everyone else"?

Well, they're distinct groups to scientists too. Nobody is saying dinosaur and bird are interchangeable terms. All birds are dinosaurs, not all dinosaurs were birds.

No one is saying that someone is saying that the terms are interchangeable :). The issue is that it’s not clear why “birds are dinosaurs” is correct but “birds evolved from dinosaurs” is not.

I don't think "birds evolved from dinosaurs" is incorrect per se, but it evokes a common misconception of what dinosaurs are, so you get people responding "birds are dinosaurs."

Consider by analogy "humans evolved from primates." Well ya, but humans also are primates.

I don’t think it’s a misconception at all, but rather science has its own definition and lay people have another. The definitions are useful to their respective camps. People who correct laypeople for using their definition are boring pedants.

I don't think that works. The lay persons definition is necessarily derived from the scientific one. "Dinosaur" wouldn't exist as a word or concept without scientific study.

Sorry to keep bringing up the same analogy, but do you also think it would be pedantic to correct "humans evolved from primates" to "humans are primates"?

The analogy doesn't apply because "primates" doesn't have a colloquial definition that excludes humans. "fish" or "reptiles" would be better examples. "Humans didn't evolve from reptiles; they are reptiles". I don't find it pertinent that the colloquial definition wouldn't exist without scientific study.

Fish and reptile aren't comparable terms to dinosaur. They started as colloquial terms and (as colloquially used) refer to groups based on traits, not phylogeny. Humans actually are not evolved from the phylogenetic group closest in content to "reptiles."

I guess you could argue the colloquial definition of dinosaur is similarly trait based, but even then... an ostrich is practically a small toothless T. rex.

I think the bird -> dinosaur relationship is more like human -> primate.

IIRC "fish" is kind of a fuzzy term. Like we would colloquially describe our common ancestor with frogs as a fish, but phylogenetically they would be closer to us than to some other "fish." Or something.

"Paedomorphosis" doesn't sound that different to the "Ontogeny recapitulates phylogeny" observation.

Adds an interesting perspective on the controversy.

Link: https://en.wikipedia.org/wiki/Recapitulation_theory

Is there a picture anywhere of the chick embryos with the dinosaur like face they mention?

https://www.smithsonianmag.com/smart-news/chicken-embryos-ge...

That one has an image.

Does this mean that distinct, non-flying species of dinosaur independently evolved to be able to fly? Or does it mean that one species of dinosaur evolved to have the ability to fly, and then further branched into all know species of birds?

According the the Wikipedia page on animal flight, dinosaurs and birds are believed to have evolved flight independently.

https://en.wikipedia.org/wiki/Flying_and_gliding_animals

Actually, according to that article pterosaurs, that are explicitly not dinosaurs, evolved flight separately from the dinosaurs/birds (with two other separate times flight evolved mentioned, in insects and in bats).

Another time it evolved independently was in homo sapiens.

I think you meant to say pterosaurs and dinosaurs (in particular, avian dinosaurs, including birds) evolved flight independently.

> “Archaeopteryx seemed to emerge fully fledged with the characteristics of modern birds,” said Michael Benton

Pun intended?

I suspect so, another one:

> Yet once those avian features were in place, birds took off.

It's almost not even a pun, just a literal statement. "Fledge" just means "to become feathered". https://www.etymonline.com/word/fledge

Insert Simpson's <"That's the joke"> meme here...

This sentence strikes me as interesting:

> (For ethical reasons, they did not allow the chickens to hatch.)

What exactly is the ethical reason? Do they suspect that the dino-faced chickens will not have a good quality of life? How do they know that? Also, who doesn't want to see a dinosaur-faced chicken?

I bet it was something like

- Do we do the experiment now (or in the next quarter at least), or do we spend a lot of time getting through ethics boards with new unique proposals that would likely take forever to get approved.

It's not a dino-faced chicken. It's a deformed-chicken-face chicken. You're not going to get something cohesive by flipping a couple genes, you're going to get something malformed.

Edit: a bit of googling suggests that, at least according to the scientist leading the project, I am wrong here:

For now Bhullar has no plans, or ethical approval, to hatch the snouted chickens. But he believes they would have been able to survive "just fine".

"These weren't drastic modifications," says Bhullar. "They are far less weird than many breeds of chicken developed by chicken hobbyists and breeders."

"The rest of the animal looked OK, but one needs to think about this carefully from an ethical point of view."

I never understood why bipedal locomotion would evolve in dinosaurs. Especially since the article says it happened well before any kind of flight or even gliding.

Bipedal locomotion is more efficient in that it takes less energy to cover a certain distance on two legs than four.

I never thought about that. I wonder why it hasn’t evolved more often then? Basically limited to humans and birds. (I guess kangaroos too?)

Many animals are occasionally bipedal. Apes and monkeys and similar of course.

But also bears and no doubt many more.

Domestic cats will stand up on their hind legs to see further.

Not of those are efficient runners in that position, though. It's a careful balancing act.

Speed vs. endurance. Depends on which is more valuable in a given environment.

Predation leads to optimising for speed. Calorie cost isn't such an issue if one is either getting a meal or avoiding being a meal immediately the sprint is finished.

Even within ancient to medieval cavalry this tradeoff existed; the horse one rode to the battle would often not be the same breed as the horse one rode in the battle.

(cavalry having arisen in the first place because a slow horse is still much faster than a fast man. infantry that can't run can't regroup)

Two critters threw a monkey wrench into that machine. We're one of them, canids are the other. Persistence hunting is a thing. Let them sprint; they'll have to sprint again when we get there. Sooner or later, they won't be able to sprint anymore.

Fast men are very competitive with horses over long distances [0]. Horses have other advantages like being able to carry more weight and live off grass.

[0] https://en.wikipedia.org/wiki/Man_versus_Horse_Marathon

> Fast men are very competitive with horses over long distances [0].

This might be true, but the citation says the opposite:

> In 2013, extensive forestry works meant the organizers had to modify the route considerably, resulting in a course of nearly 24 miles, instead of the usual route of just under 21 miles. Despite a very hot day, the longer and hillier course favoured the horses, with 2011 winner Beti Gordon comfortably beating the first man, Hugh Aggleton.

> Following a number of criticisms of the extended course in 2013 and 2014, the course for 2015 was shortened back towards 21 miles. This provided a more even match between man and equine

Men win sometimes, the fastest man is generally within 20% of the fastest horse, and the fastest man beats plenty of the horses. The fact that minor differences in length, weather and terrain are significant to the result is also more than enough to say the men are competitive.

But what the article says is that longer courses favor the horses. We can't just define "long distances" to mean "exactly 21 miles". "Humans are competitive with horses over long distances" is a terrible way to phrase the idea that the advantage of horses grows with increasing distance.

Consider that cavalry wipes the floor with infantry in terms of how much ground can be covered in a day. Don't think soldiers are selected for speed? Consider also that we have an inscription from an Egyptian pharaoh commemorating the excellence of his soldiers, as measured by the speed of their march.

33 km is much longer than I was talking about (and lends evidence to the sibling thread's persistence hunting); I'm guessing most cavalry pursuits of routed infantry would've been between 1 and 10 km.

Taking world record 5k (5000m/12.58min) ~= 400 m/min

versus a top CCI run (6398m/9.97min) ~= 640 m/min

we see that at these shorter distances the horse has a distinct advantage.

https://www.youtube.com/watch?v=IEFeZuc3J3g

Then why do four legged animals still exist? Especially animals that walk a lot. Elk, wolves, etc...

Could be misremembering but I recall reading that bipedalism in humans correlates with a narrowing of the birth canal, potentially increasing mortality during childbirth and also selecting for infants that are underdeveloped compared to other newborn mammals.

They can reach higher top speeds.

In Christopher McDougall's book about running, which I think launched the chia pudding trend, he has a story about a researcher whose theory is that bipedal motion separates breathing rhythm from gait when running. A quadruped has to breathe in time with their gait, because internal organs push forward into the diaphragm when the front feet land. According to the researcher, this is my jogging humans can stay cool while chasing quadruped prey, which are pushed to overheating and exhaustion.

I once heard that a springbok is a much better long-distance runner than other animals because it breathes automatically while it runs. The running motion directly powers the breathing motion, making it use less energy.

I can't find a quick source to back this up, though.

Dinosaurs never evolved bipedal locomotion, they evolved from older, bipedal animals and later some of them reverted to quadrupedal locomotion.

Sounds unlikely, considering crocodiles are their closest non-dinosaur relative.

Bipedal crocodilians were a thing at one point

source: https://www.youtube.com/watch?v=QsmV34Co32c

Interesting. With most land-based animals being four-legged today, it's tempting to assume that that would always have been the advantage over two-leggednes, with the exception of a few specialised long-distance runners (us, ostriches and kangaroos, basically). But maybe it's today's animal shapes that are the exception.

"Former senior writer"

One idly wonders what must be going on at Quanta that she ended up with such a title.

The article is from 2015. From the profile, it sounds like she no longer works there and the profile was updated to reflect that.

I think the original title, "How Dinosaurs Shrank and Became Birds," would do this article more justice. The title makes it seem like the article is just discussing a well known fact, but it actually goes quite in-depth.

I've re-howed it.

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