Here's a video with the birth of some chestbursters...

https://youtu.be/yf61ye6lK7I

Lifeforms don't devolve, they evolve. The evolution from resembling the adult to looking more serpentine must have be advantageous for the xenomorphs to do so. Perhaps their offspring needed to be more maneuverable and having limbs hampered them for a quick escape? 

"Lifeforms don't devolve, they evolve"

Typically this is kind of true.... Organisms can go back to a Previous Stage of Evolution if the Environmental Factors the Organism is in Favor some of the Previous Traits.

So it Technically would be to EVOLVE back to a prior Stage if this Stage is more suitable for the Organisms Current Benefit.

For Example if a Organism/Mammal had Thick Fur Coat that it Evolved into having a Small Smooth Coat as the Environment/Climate had become Much Warmer... should the Environment/Climate once again become and stay much Cooler or the Organism Migrates to a Much Cooler Environment then over the Course of Evolution the Organism could go back to its Thick Fur Coat.

So its not quite Devolution but a Organism can Revert back to a Prior Stage of Evolution if the Current Environment is more suited to that Previous Evolutionary Stage as far as Survivability

Regarding the Chest Busters...

This is a Interesting One...  IF we assume the 1979 ALIEN Organism is a Evolution then the Aesthetic Differences must play a Important Role to the Organisms Survival.

So in this Context we have to Assume the 1979 Chest Buster offers the Xenomorph Greater Survival Rate, we get the Picture a Chest Buster is a Organism Stage of the Cycle where it is at its most Vulnerable.  So the Serpent Chest Buster maybe has Speed on its side to Escape from Threats.

Something to Consider is that David has Drawings of the Previous 1979 Chest Buster, these are likely just Art Work from the Concept Artist that they thought would be COOL to add.

However such Drawings to help some to argue that the Xenomorph of Davids is NOT the Original Ancient One.

Could it be this is the AIM of David the Perfection he Foresees? 

It is just a Oversight IMO....

or we could maybe look at it as a Previous Version that David First Created, before he Evolved it to the one in ALIEN COVENANT.... but after what transpires between ALIEN COVENANT and ALIEN the Evolved ones are gone but the Previous Versions by David get onto the Derelict?

could the Natural Environment of where the Chest Buster is Gestated have a effect on the TYPE of Chest Buster?

EDIT:

I noticed that the ALIENS Chest Buster has like Arms and Hands and Legs and Feet and NOT the Pectoral Fins of the one from ALIEN

Alien Resurrection its not clear to determine if this Chest Buster has Arms or Fins or Neither, but it seems they could have Fins.... The Queen however has Arms.. just as in ALIEN 3 also.

To me, Covenant chestburster is still a prototype. It grew much more rapidly than any others seen before which looks to have started during gestation.

@Big Dave

Yeah but evolution technically doesn't have stages, it might evolve to look similar to its ancestor but its still not a 1:1 because it would have traits its ancestor lacked (such as additional vestigial organs)

I mean every time a terrestrial vertebrate evolves to become aquatic they can't re-evolve gills, they retain the lung adaptation every-time.

Also regarding to the chestburster, wasn't the original supposed to have front limbs but Ridley Scott decided to snip them off?

Typically this is kind of true.... Organisms can go back to a Previous Stage of Evolution if the Environmental Factors the Organism is in Favor some of the Previous Traits.

No. Evolution does not come back.

For Example if a Organism/Mammal had Thick Fur Coat that it Evolved into having a Small Smooth Coat as the Environment/Climate had become Much Warmer... should the Environment/Climate once again become and stay much Cooler or the Organism Migrates to a Much Cooler Environment then over the Course of Evolution the Organism could go back to its Thick Fur Coat.

It's a delusion. If the "organism" loses wool and then acquires it again - it will be another "organism". Maybe the similar, but not the same.

And further - "organism" (body) can't evolve. Only species.

@Leto That's not correct. Evolution often involves switching off genes that no longer serve a useful purpose. For example, when you, I and all other modern humans were floating inside a uterus in the first weeks of life, we had gills in our necks, a legacy from our aquatic past. We are stuffed full of genes that could be re-purposed as nature/evolution sees fit.

@hox

Oh yeah forgot embryo had gills

hox

This is not an evolution. This is ontogenesis and Recapitulation theory.

@Leto

I agree. A lot of people think of evolution like its pokemon or digimon but the fact of the matter is that Evolution never goes backwards. There is no such thing "De-evolving" because when an organism regain "primitive traits" its still a different organism than its ancestors and if you could bring said organism back into the past with its ancestor that resembles it, biologically the two creatures are completely different organisms due to the latter's evolution history.

Example: If you take a Human from the 21st Century and send him or her back to the Pleistocene where the first anatomically modern humans evolved. Biologically current modern human had went through several mutations (ex, Europeans gaining the lactose tolerance gene), survived several epidemics and non-african Homo sapiens sapiens absorbing the neanderthal into their genome while african Homo sapiens sapiens continued to evolve in their own way after the Pleistocene.

When i was talking about DEVOLUTION i was referring to that Technically this is considered Impossible, but that SOME Organisms can Revert to Previous Traits of their Ancestors that they had lost over time, this happens if these Traits are suited to Environmental Changes and Survival and in these cases its NOT a case of Devolution but more of a Re-Evolution.

Technically when this happens they could be considered a NEW Species because while they could gain some LOST Traits, they would also keep some of the Previously Gained Traits from Evolution.

So i should have detailed more as when i put "Organism can Revert back to a Prior Stage of Evolution"

I meant as i mentioned earlier in that post that SOME TRAITS can be regained, and i NEVER implied that a Organism would 100% go back to a previous Stage.

Even if a Organism regained a Number of Traits Lost in Evolution, it would maintain a Number of Traits it gained from the Process of Previous Evolution.. so i do apologize i should have elaborated a bit more.

So my comment "

"Lifeforms don't devolve, they evolve"

Typically this is kind of true"

I meant as in its True as far as Lifeforms Evolve and dont Devolve, but then to add that a Organism could gain Previous Traits Losts....  but as i further added this is NOT any kind of Devolution however.

so the Typically Kind of True was the Incorrect use of Words by me....   I should have put (as i meant) this is Technically True but some Exceptions Organisms can revert to previously Lost Traits but this would not be Devolution... 

Because when this happens its so these Traits of of Benefit and anything that is Changed for a Improvement/Benefit is a Evolution.

So to carry that on in Context to the OP.

Then it depends HOW they reveal the Origin of the Xenomorph,  because if we ASSUME as indicated that David's Chest Buster from Oram is a Predecessor to the 1979 Xenomorph than we have to assume the loss of Limbs in Chest Buster Stage is due to some Survival Benefit.

But there are TWO things to consider in Context to the Chest Buster...

1) This is purely Sci-Fi....

2) We are talking about a Engineered Creation.

So when we are talking about Genetic Manipulation to the Advanced Level Implied within the Franchise then Genetically you could DEVOLVE a Organism.

As far as Advanced Genetic Engineering goes... but NOT as far as Natural Evolution.

I got what your trying to say but the terminology was what was bothering me. Evolution is evolution no matter if its a re-evolve trait or not, its still just called evolution.

At any rate, both chest bursters from the OP came from humans but were different. So it seems the face huggers must have been from different genetic stock? The Runner and Deacon were pretty much fully formed but smaller when they burst. Same can be said for the back burster- not really sure about the throat burster though.(evolve/devolve- either way it is a change).

I think the Xenomorph is a Special Case/Organism that actually Evolves while Gestating to Adapt to its Environment.  So while some Organisms could Re-Evolve some Traits it predecessors had if they Benefit its Changing Environment with all EVOLUTION these things take  a LONG time to Evolve.

The Xenomorph seems to be able to Adapt/Evolve while Gestating and so has a Rapid Evolution and who knows if when they are ADULTS they cant still Adapt Further.

The Differences in Chest Buster could be from Different Stock as Certainly its implied the Xenomorph in Alien Covenant has some way to Evolve, but also throughout the Franchise the Chest Busters have also varied a little too.

Ignoring AVP and only using the Movies to consider this Next Point..

ALIENS we see the Chest Buster has Limbs and not Fins... in ALIEN it has Fins, and in Alien Resurrection it is hard to make out but more likely has Fins (apart from Queen), in ALIEN 3 the Chest Buster is from a Dog/Cow and has Limbs.  And so does Orams in Alien Covenant

LIMBS: Aliens, Alien 3, Alien Covenant.

FINS: Alien and Alien Resurrection?

Gestate/Birth on a Habitable World:  Aliens, Alien 3, Alien Covenant.

Gestate/Birth on a Space Ship in Space: Alien and Alien Resurrection?

But maybe thats a Coincidence... we never saw the LOPE Chest Buster if we had and it had FINS then we could maybe make a Connection.

I cant see the Reason why Gestating/Birth on a Ship or on a Habitable World (Natural or Terra-formed) would determine if they have Limbs or Not However....

So we could all try and work out what the differences are and WHY but they could all just be Creative Aesthetic that the Production has chosen.

The thing that could make SENSE for them having the Formed Limbs on a Open World compared to the Confines of a Ship with Fins could be that in the Confines of a Ship, the Snake-like Chest Buster is Beneficial for the Organisms to Escape and Hide... because no matter how FAST you are, when your in the Confines of a Ship eventually you could get Caught!

Well originally the chestburster in ALIEN had limbs but Scott had them removed. Hence why it look like it has fins because those where the remains of its shoulders while Alien Resurrection's queenburster was inconsistent with ALIEN 3's Queenburster. The Alien Resurrection Queenburster resembled a traditional chestburster somewhat while the ALIEN 3 Queenburster resembled a miniature Queen.

sorry i couldn't find a better image of the Resurrection Queenburster

It's tough to get a good shot of the Resurrection burster but it pretty much looks like the OG burster while being noticeably smaller It also had an umbilical cord which may or may not have been in the original? 

except the Resurrection chestbursters have a more snake-like mouth than the original chestburster

Also, with that very brief scene, that Resurrection burster initial hiss appeared a bit like a penis head emerging from its foreskin- Giger must have been proud: check at 1:35

https://www.youtube.com/watch?v=zCqoA-FGgoc

Yeah I prefer the queenburster from ALIEN 3, simply because she is very distinct from the common chestbursters.