Download Abstract The platypus is one of Earth`s most perplexing

Abstract
The platypus is one of Earth’s most perplexing creatures. It is unquestionably a mammal,
but shares many similarities to reptiles, and even some to birds. Though the platypus might seem
to be a random amalgamation of traits, its existence actually highlights the complexity and
elegance of evolution. The platypus is a highly adapted organism that has evolved to occupy its
own niche. Monotremes, the small group of egg laying mammals that includes the platypus and
its cousin the echidna, survived in Australia because they were better adapted to aquatic
environments than marsupials. The platypus’s odd skeleton and ability to lay eggs are not just
remnants of a reptilian ancestor, but are actually incredibly adaptive traits that allowed the
platypus to successfully survive even when competing with marsupials. In fact, many of the
reptilian elements have been retained in the platypus because they are adaptive to its unique
challenges of life as an amphibious mammal. The platypus has also evolved some unique traits
of its own, such as electroreceptors in their mouths. Every odd trait that the platypus has helps it
thrive in its environment, and the very existence of the platypus provides strong evidence for
evolution.
Keywords: Platypus, Evolution, Reptiles, Adaptation, Natural selection
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An Argument for Evolution: The Case of the Platypus
When the first specimen of a duck-billed platypus was brought back to England from
Australia in 1798, it was actually thought to be fabricated (Hall 1999). The odd mixture of avian
and mammalian traits made it seem as though this animal could not have been real. At first
glance, the duck-billed platypus does indeed seem like an odd mixture of traits. It has a beak and
webbed feet like a duck, and yet has the soft fur characteristic of mammals. And considering that
it also lays eggs, it is no wonder that scientists debated greatly over the classification of the
platypus. In fact, even its scientific name, Ornithorhynchus anatinus, meaning paradoxical bird
snout, shows the confusion of the scientific community at the time (Hall 1999). Today, it is well
established that the platypus is unquestionably a mammal, because of its fur and the fact that it
has mammary glands. However, the platypus has retained an odd amalgamation of reptilian,
mammalian and even some avian characteristics over the course of its evolution, just as early
scientists thought. In this essay, I will discuss how the platypus’s distinctive adaptations and its
retention of non-mammalian traits offer convincing evidence for evolution. Its seemingly random
combination of traits from different lineages of animals gives evidence for evolution because it
shows the platypus’s ancestry in animals with these different traits. In addition, its mixture of
traits is actually far from haphazard, because the platypus’s many unique adaptations have
allowed it to survive competition and thrive in its environment, thus providing more proof of
natural selection at work.
The lone species of platypus alive today is found exclusively in Australia, where its
unique adaptations have allowed it to thrive. Over 250 million years ago, the single land mass on
Earth was Pangaea, which split over time into two portions, Laurasia and Gondwana. The
ancestors of monotremes, the small group of egg laying mammals that includes the platypus and
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its cousin the echidna, developed on Gondwana, which later split into modern day South
America and Australia, among other continents. In the South American environment, the
monotremes died out, but in Australia, they survived (Nature 2008). That the monotremes were
able to survive in the more sub-tropical Australia and not in South America, whose southern tip
always remained in areas of cold temperature, shows that natural selection of animals differs
based on environment (Pascual et al. 1992). The monotremes were better fit to survive in
Australia’s environment than South America’s, explaining why they are found only in Australia.
Just looking into the geographic distribution of the platypus provides evidence for evolution
based on the different selection pressures of different environments.
Monotremes, and the platypus specifically, have survived in Australia because of their
unique adaptations that allow them to compete with the marsupials. The monotremes dominated
Australia after the split of Gondwana until the marsupials came to Australia, anywhere from 71
million to 54 million years ago (Choi 2009b). These marsupials took over, as they were more
mobile and better able to take care of their young, who were born live, than the egg laying
monotremes (Choi 2009b). Monotremes seem like they would have been wiped out completely
by these supposedly superior marsupials, but they were able to survive by finding their own
specific niche, where they were better adapted for life than marsupials. This niche was water
(Choi 2009a). Those monotremes that could live in the water did not have to compete with
marsupials, which were less fit to survive in aquatic environments. Marsupial offspring have to
stay in their mother’s pouch for a great deal of time after birth so they can drink milk.
Monotremes, on the other hand, lay eggs, and their offspring do not have to constantly drink
milk (Choi 2009a). In the water, young who have to constantly drink their mother’s milk are at
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the risk of drowning, giving monotremes the clear advantage when it comes to aquatic
environments (Choi 2009a).
The platypus’s ability to lay eggs is not just a remnant of a reptilian ancestor, but is
actually an evolutionary adaptation that has given them their own unique niche and allowed their
survival. The monotremes who were able to live in the water are the ones who survived and gave
rise to the modern platypus and terrestrial echidna, which evolved from the aquatic platypus
(Phillips et al. 2009). Platypuses are the only species in their genus, which means that of all the
other monotremes living in Australia, only this one lineage of platypus survived. They survived
because in addition to laying eggs, they had traits that allowed them to thrive in the water
(Phillips et al. 2009). This is a striking example of natural selection at work, because only those
monotremes that had adaptations allowing them to live in the water and avoid competition from
marsupials were able to survive.
Other specific reptilian traits have also been retained in the platypus, some to improve
their ability to survive in water, and others simply as reminders of reptilian ancestry. These traits
are especially interesting, because the fact that platypuses even have these characteristic traits of
a completely different lineage gives concrete proof of evolution. For instance, platypus shoulders
are similar to those of reptiles, and though they are not well adapted to running, they allow
platypuses to have the large shoulder muscles required for swimming (Choi 2009a). The reptilian
skeleton maintained by the platypus is indicative of natural selection, because this particular
structure has been selected for and retained due to the increase in survival and reproduction that
likely goes along with better swimming ability. The reproductive system of platypuses is also
very similar to that of reptiles. Aside from the obvious characteristic of laying eggs, platypuses
also have a cloaca, a single opening for the digestive, excretory and reproductive tracts, which is
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found in reptiles, but not mammals (Hall 1999). This reptilian reproductive system has obvious
survival advantages, as noted in the previous paragraph.
Other traits, though they may not have any survival advantage, are obviously indicative
of a reptilian ancestor as well. They have been retained simply because there has been no
selection pressure for or against them. Male platypuses have internal testes and thin, elongated
sperm with a threadlike head, much like reptiles (Griffiths 1988). Platypuses also have reptilian
eyes (Warren et al. 2008), and certain aspects of their genome itself show similarities to those of
reptiles (Brown 2008). The presence of these reptilian features in this mammal provides concrete
evidence for evolution because they show that mammals have some shared ancestry with
reptiles. While placental mammals and marsupials have evolved further and no longer show
these reptile-like traits, their existence in the platypus is indicative of the evolution of mammals
from a reptilian ancestor.
Male platypuses also have the ability to produce venom similar to reptile venom. This
trait is particularly interesting, because it too provides a clear example of evolution and natural
selection at work. During mating season, male platypuses have the ability to produce very potent
venom that is stored in a hollow spur on their back legs. In fact, this venom is strong enough to
kill a dog or severely harm a human, incapacitating them for three or four weeks (Nature 2008).
This venomous system has evolved as a way for male platypuses to assert their dominance when
confronting other males, which explains why it is only active during mating season (Whittington
et al. 2008). This seasonal inactivity is in itself an example of natural selection, because while
having a constantly active venomous system would seem to be the most beneficial for selfdefense, having the system active only when required during mating season actually conserves
valuable energy that can be used elsewhere in order to increase survival.
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The venom is very similar to the venom found in some species of snakes, but further
protein analysis shows that the two venoms have evolved separately due to convergent evolution
(Whittington et al. 2008). Certain similar antimicrobial proteins have evolved into the toxic
venom proteins not because of shared ancestry, but because these proteins are predisposed to
evolution into venom (Whittington et al. 2008). This characteristic of platypuses adds another
layer of intrigue to its evolutionary story, because although this venom seems at first glance to be
characteristically reptilian like many of its other traits, it actually is a result of independent
evolution that has simply ended up at the same spot as in reptiles. The fact that they have not
developed from a common venom highlights the peculiar ways in which evolution works. It
would probably make more sense if the similar venoms were a result of shared ancestry, like the
other reptilian traits. But evolution does not make sense, and is instead idiosyncratic, acting only
on whatever variation exists at a certain time. The variation that led to venom development
existed only after reptiles and monotremes split, and thus the venoms developed independently
of each other. It is odd examples of trends like convergent evolution that provide some of the
best evidence for evolution, because they highlight the mechanisms through which evolution
acts, namely natural selection.
Adding another layer of evolutionary interest to the platypus is the fact that they also
share some common characteristics with birds. Though the bill of the platypus is not avian, as
early scientists once proposed, there are aspects of the platypus genome that are shared with
birds. The platypus is unique in its method of sex determination, because it has 10 sex
chromosomes, while birds and other mammals have just two (Brown 2008). The platypus’s sex
chromosomes follow an XY system, like other mammals, which means that males have an XY
pairing, while females have an XX pairing. For example, a platypus with five X and five Y
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chromosomes is a male. Birds are the opposite, with a ZW system, meaning that males are ZZ
and females are ZW (Veyrunes et al. 2008). While it seems like the XY systems of the
platypuses and its mammalian relatives should be closely related, it turns out that the platypus X
chromosomes have the most similarities with the avian Z chromosomes (Brown 2008). This
shows that platypuses have evolved their XY sex chromosomes from an ancestor with a ZW
system like that of birds, and that the sex chromosomes of other mammals evolved
independently after they split from the monotreme lineage (Veyrunes et al. 2008).
The retention of the avian like sex chromosomes in platypuses proves descent from a
non-mammalian ancestor. That the platypus has ten sex chromosomes compared to two in other
mammals can only be explained in light of evolution as well, because the platypus system and
the mammalian system evolved independently from one another, after the two lines had split
from one another (Veyrunes al. 2008). It would seem most logical for all mammals to have
similar sex chromosomes, but evolution does not follow any kind of logic. Evolution and its
strange ways are the only way to explain the differences in the bird like sex chromosomes of
platypuses and those of other mammals. Other similarities between birds and platypuses can also
be found in their genes. The genes coding for egg yolk proteins in platypuses are very similar to
those found in chickens (Nature 2008). The platypus also has two genes related to fertilization
that are only found in birds, amphibians and fish, but not other mammals (Warren et al. 2008).
These shared genes are further evidence for evolution, because they prove that platypuses have
come to be by descent from ancestors with avian-like traits. There is no other way to explain
these homologies between two very different types of animals.
Despite all of these similarities with birds and reptiles, platypuses are definitely
mammals, showing their evolution along with other mammals after the split from the early
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reptilian ancestor. Overall, platypuses share 82 percent of their genes with other mammals,
showing that they are unquestionably mammals. (Warren et al. 2008). Shared traits with
mammals include the obvious fur, as well as microtubules in platypus sperm and its large
macrochromosomes that are characteristic of mammals (Griffiths 1988). Females are able to
lactate like all other mammals, and they share the same milk proteins (Warren et al. 2008). These
are just some of the many traits shared among all mammals, and they show that the platypus has
evolved on the mammalian lineage, despite their retention of reptilian characteristics. The
common ancestor of mammals had some reptilian traits, and when monotremes split from the
other mammals, they ended up retaining those traits along with the unique mammalian traits it
had evolved (Penny and Hasegawa 1997). The platypus’s distinct mix of mammalian and
reptilian traits is indicative of evolution, because evolution is the only way to explain why the
platypus, a mammal, would even have these traits.
Platypuses and marsupials specifically are most closely related, which explains many of
their similarities (Penny and Hasegawa 1997). These similarities also provide evidence for
evolution because platypuses and marsupials have retained these traits from their common
ancestor because of the specific needs of their essentially helpless young, which finish much of
their development after birth (Griffiths 1998). Platypus milk, like marsupial milk, contains high
concentrations of iron because newborns of both types have small livers that cannot store enough
iron on their own (Griffiths 1988). If this adaptation had not evolved, monotremes and
marsupials likely would have gone extinct, because their babies would be iron deficient and
unable to survive. This adaptation to the specific needs of their newborns shows natural selection
at work, selecting for those animals whose small adaptive variations led to these unique
characteristics. Both marsupials and platypuses also have an increased number of genes coding
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for antimicrobial proteins, which have evolved in order to help their underdeveloped young
combat more diseases. Placental mammals have lost these characteristics because their young are
born more fully developed than those of marsupials and monotremes (Warren et al. 2008). The
selection for these traits to address the specific needs of monotreme and marsupial young, as well
as the lack of these traits in placental mammals, provide evidence for evolution by showing that
only the mammals with the specific need for these traits have retained them over the course of
evolution from a common mammalian ancestor.
Shared reptilian, avian and mammalian characteristics aside, the platypus has also
evolved some of its own very unique adaptations to help it survive, and these adaptations are
clearly the products of natural selection. Some traits have evolved to help the platypus adapt to
the distinct challenges of living in both water and land. The platypus has a very specialized
system of thermoregulation that helps it survive cold water. It increases its metabolic rate as the
temperature drops, and its fur is better at insulation than most other mammals, making platypuses
better at maintaining internal temperature than many other amphibious placental mammals
(Griffiths 1988). This specialized system of thermoregulation is yet another example of the
highly specialized adaptive traits that have been selected for over the course of platypus
evolution. Platypus feet have retractable webbing, allowing them to swim in water as well as dig
their burrows with ease (Griffiths 1988). This trait indicates that natural selection favored those
animals that could retract their webbing in order to build better burrows, which is where they
raise their young (Griffiths 1998). These unique adaptations are proof of evolution because they
show the ways that natural selection has acted to select for those animals with the adaptations
that increase chances of survival and successful reproduction.
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Perhaps one of the most advanced adaptations of platypuses is that they have receptors in
their bill that allow them to detect their prey. While under water, their eyes and nose close,
leaving them blind to their environment. In order to detect their prey, the platypuses have
evolved chemical receptors that detect chemicals in the water entering the back of their mouths
(Nature 2008). They also have electroreceptors in their bills that detect weak electric fields
generated by their prey (Griffiths 1988). This is one of the platypus’s most intriguing adaptions,
especially because it would probably make more sense to develop eyes that can see underwater
rather than a whole new complex system of receptors. But evolution is not logical, and acts upon
whatever variation exists in the population. The existence of a variation that allowed for a
reproductive advantage in some ancestral monotreme happened to lead to this complex receptor
system. These receptor systems are indicative of the way that evolution does not act towards any
kind of perfect adaptations.
In conclusion, the platypus is more than just an amalgamation of random reptilian and
mammalian traits, but is instead a highly adapted organism that has evolved to occupy its own
niche. The mixture of all these traits from different types of animals is clear evidence for
evolution. Only through an evolutionary lens can these traits be explained, because otherwise the
odd combination of reptilian and mammalian and avian traits would seem haphazard. In fact,
many of the reptilian elements have been retained in the platypus because they are adaptive to its
unique challenges of life as an amphibious mammal. The platypus’s odd skeleton and ability to
lay eggs are not just remnants of a reptilian ancestor, but are actually incredibly adaptive traits
that allowed the platypus to successfully survive even when competing with marsupials. This
retention is actually quite fortunate for people who are looking for proof of evolution, because
they clearly show that platypuses, and thus all mammals, have descended from an ancestor
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shared with reptiles. The confusion of those first scientists when they first saw the platypus is
perfectly understandable, because at first glance, the platypus’s mixture of traits is definitely
perplexing. When examined in an evolutionary light, though, the platypus begins to make sense,
and emerges as an amazingly complex and adapted animal. As the famous quote from
Theodosius Dobzhansky goes, “Nothing in biology makes sense except in light of evolution.”
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