If the best answer to “which came first: the chicken or the egg?” is that the first bird that could accurately be called a chicken, having achieved that final mutation that separates chickens from their nearest non-chicken ancestors, hatched from an egg that was laid by a bird that evolutionarily was not quite a chicken . . .
. . . I would guess that the very first mammal was born as a mutated evolutionary variant of a non-mammal parent. How’s my reasoning so far?
Following this line of logic, if one of the qualifiers for being designated a mammal is that the young feed from milk produced by the mother, I would imagine one of two possible scenarios for the mysterious Mammal Alpha:
[ol]
[li]The first mammal was born of a “pre-mammal” who had evolutionarily mutated heretofore “useless” mammary glands, thus the first mammal fed from the milk of the pre-mammal mother.[/li][li]The first mammal really should have been feeding on mother’s milk, but as the mother had no milk (not quite being a mammal), Mammal Alpha toughed it out through infancy surviving against all odds until, as an adult it could produce milk to feed Mammal Beta (Gamma?).[/li][/ol]
So, how did it really happen?
P.S. I know that evolution doesn’t really work this way, that it happens slowly with micro-changes over guzzilquillions of years, rather than with a “sudden” appearance of the new.
However, having not studied evolutionary science, nor zoology, in any great detail, the above scripted profoundly stupid manner of asking the question is the best I can come up with in hopes of receiving an intelligent answer from a more studied Doper.
As Exapno’s link indicates, mammary glands are thought to have originated from some other kind of skin gland, perhaps sebaceous glands or apocrine sweat glands, or perhaps some ancestral form of these glands. One idea is that the original gland may have secreted a substance that helped protect the incubated egg or hatchling from microbial infection. Young may have occasionally lapped up this material and gained some nutrition from it, eventually resulting in the evolutionary enhancement of this function and the production of milk.
The transition between non-mammal and mammal would depend I suppose on when you define this substance to have become “milk,” but like many things in evolution this transition was almost certainly very gradual.
I might also mention that evolution does not typically happen by means of a single mutation that produces some feature that was never present before. Instead evolution happens by selection on existing small genetic variations (that were generated earlier through mutation), recombining sets of genes so that they in concert gradually enhance some feature.
I’ve heard the “modified sweat gland” theory before. Which I find bizarre as a matter of cladistics, since while there are extant non-sweating mammals, there don’t seem to be a lot of extant sweating non-mammals. Are there any sweating birds or reptiles?
Now, sebaceous glands I could see, except those are, again, generally mammalian, & connected to hair follicles.
I’d really like to see something with skin glands that’s not a mammal to verify this, but they’re unknown if not long extinct.
Many bird species, particularly waterfowl, secrete oil from skin glands to protect their feathers.
Also, fish and amphibians both have been known to secrete oils, slimes, and other stuff from their skin. Toads and poison arrow frogs are both rather well known for this.
So, from that respect, it doesn’t seem outlandish.
Cladistically speaking, this is not really an issue. The lineage that led to mammals split off before the one that led to extant “reptiles” (turtles, lizards and snakes, tuatara, and crocodilians) and the birds (which cladistically speaking are also reptiles). Therefore the glands in question can be regarded as a derived character unique to the mammalian lineage, and there is no reason that they should be expected to occur in the reptile/bird lineage. However, as has been mentioned, glands that may be homologous do occur in other tetrapods.
Although not all mammals thermoregulate by sweating, virtually all terrestrial mammals do have at least some sweat glands (exceptions being the two-toed sloth and Cape mole rat). These may be restricted to just the soles of the feet. Aquatic mammals lack sweat glands, but this is obviously a secondary loss. Sweat glands thus can be regarded as a derived character that is basal for mammals.
I don’t believe that’s true, since our “qualifiers” have to apply to fossils and we can’t be sure whether an extinct organism lactated or not. I believe that the defining characterisitc of mammals has something to do with bones–hopefully the biologists can help me out.
Defining bone characteristics of mammals include the jaws, the details about which I am fuzzy, and the inner ear bones - only mammals have the malleus, incus, and stapes, that is, three inner ear bones. There’s probably something about teeth, too, but again I’m not conversant with the details.
Mammals are also considered to have hair and to lactate. I believe it also used to include placentas and live birth, but the discovery of marsupials and monotremes required some modification of that definition. Certainly, a very old version of mammals near the split between mammals and reptiles/others may lack some characteristics of modern mammals of any sort. Purely speculating, the “first mammal” may have been a non-lactating egg-layer with hair and three inner ear bones - or it may not. Some things the fossil record is highly unlikely to tell us.
It depends on for what period you are doing the defining. All modern mammals lactate and have hair, so these can be used as defining characteristics for the class Mammalia. They also share certain skeletal characteristics.
Of course, mammary glands and hair don’t fossilize, so in practice it is necessary to rely on skeletal characteristics. Paleontologists have selected the kind of jaw articulation as the defining character. In ancestral tetrapods and in “reptiles” as I outlined them above, the lower jaw is made up of several different bones. In mammals the lower jaw has been reduced to a single bone on each side. The two bones that formed the original jaw articulation have become reduced in size and migrated to the inner ear, joining the single ear bone found in other tetrapods to make a set of three. A new jaw articulation is formed by the remaining lower jaw bone and a different bone in the skull. There are other skeletal characters used to define the Class Mammalia, but this is the key one.
Of course, it is impossible to tell at what exact stage this lineage acquired mammary glands and hair. It is likely that the three characteristics did not coincide exactly in time.
Some paleontologists think that the acquisition of a separate decidudous dentition in juveniles (“baby teeth” or “milk teeth”) is definitive evidence that particular species fed the young on milk. Of course, milk production could have preceded this trait by some time. Also, there are suggestions that nerve canals in the snout of certain fossil species indicate the presence of vibrissae (e.g. sensitive whiskers like those of cats) and hence hair.