Saturday, March 24, 2012

Polydactyly, or Having More Than Five Digits on the Distal Portion of a Limb


            Having an abnormal number of digits occurs more frequently than you might think, 1 in every 500 births, and it’s been going on for hundreds of years. The most famous case in history is that of Anne Boleyn. It was rumored she always wore gloves because she was hiding a sixth finger. This was seen as a sign of a witch, and it was one of the charges read against her before her beheading (amongst the charges of adultery and incest). The fact that an extra appendage was used in 1536 to identify evil meant there were prior occurrences of the same trait. Would an extra finger be more helpful or more cumbersome? Which way would natural selection act?
            Surprisingly, polydactyly is an autosomal dominant trait, but the allele frequency is now so low that it is a rare condition. Another theory suggests that polydactyly is the result of mutations in a variety of developmental genes in an individual (i.e. not heredity). One gene thought to be affected is the LMBR1 gene, or limb region 1 protein homolog. It produces a protein used in regulating developmental gene expression of the sonic hedgehog gene. This gene makes proteins for the hedgehog signaling pathway in mammals that plays a role in organogenesis, especially the growth of digits on the distal parts of limbs. If the LMBR1 gene is mutated, it can interrupt expression of the sonic hedgehog, affecting limb patterning.
Although classified as an extra finger, the digit often doesn’t have a joint, but rather is an abnormal fork from an established digit, thus making it a non-functional protrusion of soft tissue and some bone. Thus, it is often removed at birth. With no joint, the extra digit is useless. With a joint, however, it might be useful, and one might think natural selection would favor this extra, functional finger. Yet, the occurrence of a functional joint is rarer than an extra flab of flesh. Perhaps this is due to a developmental gene needing to be kept “on” longer than normal, and the longer it’s kept “on” by a mutated expression gene, the closer the extra digit gets to a functioning digit. Since normal development calls for this expression to cease early on, expression will shut down before a fully functional additional digit can be formed, leading to a non-functional appendage more often than not.
            At this point, we would think natural selection would select against a useless flap of skin. Without a joint, it might be more of a hindrance to daily life. Humans helped selection with their ignorant interpretation of additional digits. Physical anomalies were seen as physical manifestations of evil, so oftentimes, these people would be shunned or put to death, both acting as crude forms of sexual selection because that “deformed” person would not get the chance to spread his/her genes to posterity.
            In human populations, polydactyly is seen frequently among small communities in India and the Old Order Amish in Pennsylvania. In the Amish population, polydactyly isn’t an independent occurrence, but a symptom of Ellis-van Creveld syndrome (a type of dwarfism), a genetically inherited disorder. There is such a high occurrence of this disorder (amongst others) in the Amish population because they tend to marry within their community for generations, preventing new alleles from entering the gene pool. This combined with genetic drift (random fixation or deletion of alleles over time in an isolated population) and the founder effect decreases genetic variation in this population.
            This condition, however, is not unique to humans. It can be seen in small mammals, such as dogs, mice, cats and moles. Talpid moles were thought to have six digits on each foot, but in actuality, it is an outgrowth of the radial sesamoid bone, or a wrist bone. Like the human flaps, this “digit” although it has bone, does not have a joint, so it can’t be a real finger. The development of the extra digit occurs during development after the five true fingers develop, giving another possibility to the development of the anomaly in humans. Natural selection has put positive pressure on this type of mole because the sixth digit assists in digging, so the talpid mole is quite common. In humans, however, we don’t need to dig and there is no inherent advantage to an extra digit, so it appears natural selection is working in reverse compared to its operations with moles.
            Our fascination with extra digits lead to the exhumation of Anne Boleyn in the reign of Queen Victoria (late 1800s). Anne, in fact, did not have polydactyly. It was just another weak accusation to depose her so Henry VIII could move onto another woman. Although the origins of polydactyly are not specifically known, it’s very much around, since it’s hard to get rid of traits/alleles (if, indeed it is hereditary) with low frequencies.

Sources:
·         http://rsbl.royalsocietypublishing.org/content/early/2011/06/29/rsbl.2011.0494.abstract, Mitgutsch, Christian et al., “Circumventing the polydactyly ‘constraint’: the mole’s ‘thumb’”
·         Heus HC, Hing A, van Baren MJ, Joosse M, Breedveld GJ, Wang JC, Burgess A, Donnis-Keller H, Berglund C, Zguricas J, Scherer SW, Rommens JM, Oostra BA, Heutink P (Aug 1999). "A physical and transcriptional map of the preaxial polydactyly locus on chromosome 7q36". Genomics 57 (3): 342–51. http://www.sciencedirect.com/science/article/pii/S0888754399957968
·         Ianakiev P, van Baren MJ, Daly MJ, Toledo SP, Cavalcanti MG, Neto JC, Silveira EL, Freire-Maia A, Heutink P, Kilpatrick MW, Tsipouras P (Jan 2001). "Acheiropodia is caused by a genomic deletion in C7orf2, the human orthologue of the Lmbr1 gene". Am J Hum Genet 68 (1): 38–45. www.sciencedirect.com/science/article/pii/S0002929707624707
·         http://www.dailymail.co.uk/news/article-2018470/Akshat-Saxena-Indian-boy-born-34-fingers-toes-breaks-world-record.html “On-year-old Indian boy breaks world record after being born with thirty-four fingers and toes.”


Wednesday, March 21, 2012

The vestigial human tail

Every once in a while I hear a story of the possibility of a human being born with a tail. I hear a lot of different suggestions as to why this occurs, but my plan for this post is to find the truth behind these reports. If the reports are true and some humans are born with tails, I will them explore why our recent ancestors stopped developing tails, and whether or not the gene still exist in humans.

So when people think of the possibility of a human developing a tail, they usually picture something like this (3).


The reality is that human tails do exist but they hardly look like this!
Some real examples of human tails are as follows (2 & 3).



Normally, a tail is present on the developing human fetus, but usually regresses by the 8th week of development. The true human tail upon birth is caused by a lack of cell destruction of the distal end of the embryonic tail (1). These "true humans tails" are composed of adipose tissue, connective tissue, muscle tissue, various nerves, and blood vessels (like any other true tail) and ranging in size from one to more than 5 inches long. There does exist a spectrum of structure with these "true human tails." While the majority of "true human tails" have neither catilage or developed vertebrae, there have been cases of newborns possessing a "true human tail" with 5 developed vertebrae (1 & 2).

Though what is described above is the true human tail, there is such things as a pseudo tail that accounts for at least one-third of all reports of human tails. These pseudo tails do not develop from the lack of regression of the embryonic tail, but rather arise from complications such as in Spinal Bifida, various lesions, or due to an elongated parasitic fetus (1 & 2).

Now that we know that the "true human tail exists," my nest question is whether or not the "true human tails is coded for genetically or is due to derailed chemical signaling during development. I found the answer in an article of vestigial traits by talkorgins.org . Their article on "Evidence for macroevolution" states "true human tails" as an example, and we all now that in order to be an evolving trait, you need to have the present molecular biology, or gene. The article acknowledges a paper by Standfast that accounts three generations of females inheriting a "true human tail." The article also presents 2 papers by Katoh and Roelink, who discovered that the same genes responsible for tails in mice are also present in Humans. These genes are Wnt-3a and Cdx1 (1).

So my next questions while reading were as follows:
1) Do all humans at one point in their life have a "true human tail," and if so how do we lose it?
or
2) Do we all have human tails, and if so is the organ underdeveloped and unnoticeable?

Thankfully, the article provided the answers to my questions. The answer is no, not all humans have "true human tails." The truth is that all fetuses develop an embryonic tail that is then signaled for cell death, or apoptosis, by the inhibition of the Wnt-3a gene. This means that the cause for the "true human tail" is due to the unsuccessful inhibition of the Wnt-3a gene during the early stages of human development (2 & 3).

So finally I come to my last question about "true human tails." Which one of our recent ancestors was the last to have a tail? The article nicely answers this question too. Go figure! (They got their stuff down.) The article states that currently it is believed among evolutionary biologists that the "true tail" was lost during the evolution of the apes due to due to the lack of Wnt-3a gene expression (1).

But why did the evolution of the apes get rid of the "true tail?" Was the environment that the apes lived in more conducive to not having a tail and actively selecting against apes with more Wnt-3a gene expression, or was this due to a mutation that went to high frequency within populations of apes?

I hope to explore these final questions in my next blog post, but if you would like to read up on possible answers to these questions you can visit:
http://www.sciencedaily.com/releases/2011/08/110803133505.htm
http://www.quora.com/Why-do-none-of-us-Great-Apes-have-tails

Thanks for reading, and GOOD LUCK on the next test!

Estevan Delgado

--
Sources:
(1) http://www.talkorigins.org/faqs/comdesc/section2.html
(2) http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3263034/?tool=pubmed
(3) http://funstuffcafe.com/human-tail-just-nature-going-wrong

Tuesday, March 20, 2012

The Great Appendage

A new fossil species, Schinderhannes bartelsi, discovered in a German quarry 3 years ago, may provide a clue about the evolution of grasping claws. S. bartelsi was an ancient creature with large, bulging eyes and what scientists call a "great appendage."


What is a great appendage? Great appendages are large, interconnected claw-like appendages that are attached to the heads of "great appendage arthropods," which include anomalocaridids. This group consists of early marine animals from the Cambrian period. Great appendages are highly modified limbs that are thought to help the organism catch prey and manipulate food (i.e., grasping and handling).


Modern arthropods have grasping claws that appear to have evolved from the great appendage. An example is the pedipalps (i.e., pincers) of the scorpion. Also, great appendages are thought to be homologous to antennae of insects and to chelicerae (mouthparts) of Chelicerata such as spiders. Up until the discovery of S. bartelsi, evidence from the fossil record had led scientists to believe that the great-appendage arthropods had all disappeared during the Cambrian period, creating an evolutionary dead end. However, the slate deposit in which the fossil was found dates to 390 million years ago -- which is 100 million years after the group of great-appendage animals were predicted to have been eliminated to extinction.

Thus, the fossil could be a "missing link" in claw evolution and could potentially provide clues as to how extant arthropods are related to their great-appendage ancestors.


Sources:
1. Jaggard, Victoria. "Great Appendage Photo: Fossil Linked to Claw Evolution." National Geographic News. 5 Feb. 2009. National Geographic Society. <http://news.nationalgeographic.com/news/2009/02/090205-great-appendage-picture.html>
2. Kuhl G, Briggs DE, Rust J. A great-appendage arthropod with a radial mouth from the Lower Devonian Hunsruck Slate, Germany. Science. 2009;323(5915): 771-3.

Monday, March 19, 2012

Scary Beautiful Bugs


The treehopper helmet not only gives the treehoppers an other-worldly appearance but also is an example of an appendage that arose as a new feature. This is rare since the more common observation is the loss of appendages as opposed to the introduction of new features/ appendages, which is thought to add further constraint. The treehopper insects (Membracidae), a small group of hemipteran insects to cicadas, have evolved a highly diverse range of helmets, from those mimicking tree branches, aggressive ants, and other natural elements to helmets with thorns or ant animal droppings.


 It is shown that the helmet is an appendage with a flexible joint, instead of an extension of the exoskeleton. The helmet actually seems to be attached bilaterally to the thorax by these flexible joints, much like regular wings. Similarities are drawn between the anatomy of the treehopper helmet and wings. A handful of transcription factors, including nubbin, mark wing developmental fate and differentiate between wing and other appendage precursors. Nubbin was found in the developing helmet and its expression parallels that of the wings. Two other genes, Distal-less and homothorax, are also expressed in the developing helmet and determine the helmet proximo-distal axis from hinge region to posterior tip. Recently, developmental studies have shown that the introduction of the helmet has been brought about through inhibitory mechanisms that prevent the formation of wings.

Please welcome the Lady Gaga of the treehoppers.

A gene involved in inhibiting wing formation, Sex combs reduced (Scr) was thought to have something to do with the formation of the helmet. Prud’homme et al. found that the evolution of the helmet was not due to a change in Scr expression or function but rather that nubbin may have become unresponsive to Scr repression. All in all, we see evidence that helmet development may rely on developmental mechanisms involved in wing formation.

Sources:
Moczek, Armin. "The Origins of Novelty." News & Views. Macmillan Publishers Limited, 5 May 2011. Web. 18 Mar. 2012. <http://sites.bio.indiana.edu/~moczeklab/pdffiles/2011%20Moczek%20Nature.pdf>

Prud’homme et. al. "Body Plan Innovation in Treehoppers through the Evolution of an Extra Wing-like Appendage." Nature 473.7345 (2011): 83-86. Nature.com. Nature Publishing Group, 4 May 2011. Web. 18 Mar. 2012. <http://www.nature.com/nature/journal/v473/n7345/full/nature09977.html#/main>




Evolution of Fingernails

Adding onto my previous post on evolution of fingers, I thought it would be interesting to delve into evolution of fingernails. I found one post online that explains some of the hypothesis for human fingernails.

http://www.wisegeek.com/why-do-we-have-fingernails-and-toenails.htm

Fingernails consist of keratin proteins, the same material used for producing hair. The most obvious purpose of fingernails, like hair, might be protection- protection of nail beds.


After reading the post, some of the hypothesis that I narrowed down to are:


1. Fingernails resemble animal claws that other species in the animal kingdom have. Perhaps they are vestigial remains of our wild past before the usage of tools.

2. Human species consumed a lot of fruits. It is widely possible that the fingernails helped us peel fruits.


3. Fingernails might have been useful for other daily tasks, such as scratching or picking away small items, such as the behavior of apes picking each others' fleas. A modern example of this usefulness is playing the classical guitar. This preciseness allows much more detailed movements.




A good leading point is to perhaps think about the evolutionary purposes of toes and toenails. Do/did they have similar functions with fingernails, and we just lost them once we became bipedal?


Bonus picture: longest fingernails in the world- this 45-year-old woman has been growing her fingernails for 18 years that are about 6 meters long now. I'm pretty sure she doesn't play the guitar. Or pick her nose.


Sunday, March 18, 2012


Evolutionary Origins of the Hairiness of Humans

In biology, hair serves as one of the many defining traits of the mammalian family (others include: mammary glands and the middle ear).  When thinking of appendages, you would probably think of protrusions from the midline of the human body, such as arms, hands, fingers, legs; i.e. something large and thick.  However, hair is also considered an appendage of the human, with quite a few useful functions.  While some humans prefer to shave off hair in many areas of their body, they should keep in mind that hair can serve as protection from solar radiation as well as function as a social tool that aids in expression of emotion and social intercourse.  However, no matter how much we compare our hairiness with each other, we will rarely if ever surpass the furriness of our primate family.  Why are humans the only primate show such a lack of fur throughout the body that they are often described as naked?  All of our closest ancestors have fur, so why don't we?

                                          This critter is smiling because it has fur, and can still swim faster than humans
                                                        Source: http://www.flickr.com/photos/32864904@N02/4596772664/

Conjecture 1: Aquatic Ape
In 1960, Alister Hardy, an English marine biologist, voiced his thoughts that the loss of hair for homo sapiens may be related to how blubber evolved in aquatic mammals.  He hypothesized that the ancestors of humans may have undergone a phase where they adapted to a wet environment and then returned to terrestrial living before they undergone more changes that would render them completely aquatic.  However, the fact that many aquatic mammals such as otters and seals  do have dense fur sheds doubt on this hypothesis.  While dolphins and whales have smooth skin similar to that of humans, they also have much greater mass than humans.  Hair in this situation might cause these large mammals to overheat in the tropical climates that they migrate to.  The hypothesis has also been criticized for not being parsimonious, because this sort of evolution would need to explain why fur had been lost as humans were forced to aquatic environments and then need to explain why the lack of hirsutism remained as a positive trait on land.

                                         Source: Windows 7
Conjecture 2: Intense hunting leads to rapid release of heat
While Charles Darwin introduced the idea of hair loss due to thermoregulation in humans,  Dr. Peter Wheeler came up with the underlying theory behind the thinning of body hair.  He argued that with the period of global cooling, the forests became plains and savannahs, leading to much hotter and less humid weather.  With the lack of abundant shade, and the need to travel farther distances to hunt prey, a dense heavy coat would cause humans to overheat and prevent perspiration.  The prevalence of hairless humans also include an overlap with the Endurance Running hypothesis that explains the reason for certain human traits are due to adaptations to long distance running.   However, how come humans are still the only hairless species on the savannah?  Many animals such as cheetahs and baboons live just fine on the plains.  Advocates for the hypothesis point to the study that states bipedal hominids actually show less water loss when the naked skin is exposed to the African savannah.
                                                       This female can be seen as a suitable mate.  She has no ticks.
                                                                         Source: http://www.drclay.com/wp-content/uploads/2010/04/female-back.jpg

Conjecture 3: Ectoparasitism
The last hypothesis introduces that the loss of hair in humans as a counterattack to the prevalence of parasites (and the diseases they bring).  The thinning of hair would allow for faster and easier removal rate of harmful parasites on the human body.  The hypothesis becomes more fleshed out when we learn that at this point in time, humans had already established a primitive culture, found fire, and had started creating clothes.  These three factors would allow for humans to retain body heat while allowing the selection for thinner hair to continue.  Further selection for thinner hair in humans may also be due to sexual selection; potential mates with thinner hair and smooth, unmarred skin could openly exhibit their lack of parasitic infection to potential mates.   In an interesting side note, naked mole rats live in huge, underground colonies where the parasites could be easily transmitted.  However, due to the hairlessness of these critters, they do not suffer from parasitic problems due to the easy detection of foreign objects on their bodies. Researchers have hypothesized that a head full of hair allows for insulation of the part of body that loses heat the fastest, and the abundance of hair in the nether and axilla regions allow for pheromones to linger further., but no concrete studies have been conducted.

Each hypothesis discussed have their strengths and weaknesses with no single hypothesis  completely dominating the discussion due to lack of conclusive findings.  The above hypotheses discussed above only serve to act as a very general overview of the reasons for the nakedness of humans.

Sources
Bhattacharya, Shaoni (2003), “Early Humans Lost Hair to Beat Bugs,” New Scientist, [On-line],URL: http://www.newscientist.com/news/news.jsp?id=ns99993807. 

Morgan, Elaine (1997). The Aquatic Ape Hypothesis. Penguin. 

Wheeler, P (1984). "The evolution of bipedality and loss of functional body hair in hominids". Journal of Human Evolution 13: 91.  

Tuesday, March 13, 2012

Why did snakes evolve to be legless?


For millions of years, animals have enjoyed the many conveniences of transportation which legs provide; yet, there are some which travel without legs or even fins. One representative of these appendage-less creatures is the suborder serpentes, which encompasses the animals more commonly known as snakes (“Phylogeny of Snakes”). Now, why would snakes evolve to be without such a useful adaptation? Some argue that it was God’s punishment for misleading Adam and Eve. However, evolutionary biologists have other ideas.

It is a common consensus amongst evolutionists that snakes evolved from lizards; however, exactly how is still up for debate. The actual genetic reason behind the absence of legs has been found in a mutation in the expression of Hox genes which are involved in the development of different types of vertebrae. This mutation led to the formation of more thoracic vertebrae, which have ribs, than other types of vertebrae which allow for the formation of limbs (Gilbert). The genetic component has been determined. The reason for the push in that evolutionary direction, however, is still disputed.


While some scientists believe this lack of limbs stems from terrestrial origins, others insist it has an aquatic basis. The functional argument behind the terrestrial side is that when burrowing, limbs may hinder movement (Gyekis). However, some argue that the lack of homoplasy amongst other burrowing animals hampers this idea. After all, front limbs would certainly help in the removal of dirt while burrowing. Yet, paleontological evidence shows that the forelimbs were actually the first to go (Viegas). On the aquatic side, scientists argue that the skeletons of snakes closely resemble those of mosasaurs, marine lizards of the Cretaceous era that are related to Komodo dragons. Skeptics of this theory point to genetics. They argue that the snake genome most closely resembles the genome of terrestrial lizards. Additionally, to counter the questioning of the functional argument, terrestrial origin proponents propose that the snake’s ancestors may not have been creating their own burrows, but rather, invading the burrows of small prey. In this case, limbs could be very much in the way when trying to squeeze down a tight burrow to get a tasty morsel of food (Gyekis).

Personally, this author sides with the terrestrial arguments. The functional explanation and genomic evidence are much stronger than skeletal similarity. However, despite this, the consensus is far from unanimous. Thus, the “why did snakes evolve to be leg-less?” debate lives on.

Sources:
Gilbert, SF. “Hox Genes: Descent with Modification.” http://www.ncbi.nlm.nih.gov/books/NBK9978/
Gyekis, Joseph. “When Did the Snake Lose Its Legs?” http://www.rps.psu.edu/probing/snake.html
Viegas, Jennifer. “How Snakes Lost Their Legs” http://news.discovery.com/animals/snakes-lost-legs-evolution-110207.html

The Evolution of Appendage Lengths: Revisiting Allen’s Rule

If you’ve taken an introductory biology course, it’s likely you’ve heard of Allen’s Rule. According to this rule, endotherms in warmer climates usually have longer appendages than those in colder climates. The concept is simple: a higher surface-area-to-volume ratio is more conductive to heat exchange. Therefore animals from colder regions will theoretically find it easier to regulate their temperature with shorter limbs and appendages, while animals from warmer regions will benefit from longer limbs. General trends in accordance with Allen’s rule have been found in the legs of seabirds and toucan beak lengths, among others. 

Allen’s Rule is very nice and neat. Animals adapt to their environment! Evolution is fun, kids! Rainbows and unicorns! Unfortunately, it doesn’t capture the full complexity of appendage length determination. 

First of all, Allen’s Rule does not apply in all cases. Studies on American rabbits and hares show that appendage lengths do not necessarily follow the rule, even though hares' ears are often given as an example of how a higher surface-area-to-volume ratio has a beneficial cooling effect. 

Limb length is also partially determined by environmental factors, and does not necessarily result from purely genetic changes. In fact, temperature changes on their own have been shown to modulate cartilage growth, one of the main factors determining bone growth. Not only do mice raised in colder environments show marked decrease in tail length, but metatarsal bone cultures also show expansion independent of “mechanical, dietary, vascular and systemic hormonal influences” – that is, under the influence of temperature alone. 

Though all this may seem somewhat 'anti-evolution' for a blog specifically devoted to evolution, it emphasizes the dangers of quick solutions to patterns we see in nature, and of automatically assuming evolution without a genetic basis.

It also makes me wonder about whether it is possible to still read evolution in this tale of appendage length development. The experiment above only suggests that Allen’s Rule could largely be explained by purely environmental factors, not that genetic factors are necessarily non-existent. Could phenotypic plasticity play any role? When would a hypothetical new gene regulating appendage length persist? After all, it seems that any gene that works to perform something the environment does “for free” would incur some sort of fitness cost to the organism. What about the cases where genetic regulation of appendage length would be beneficial, perhaps when limbs are used for flying or swimming? 

Just to add to the confusion, here’s one final study. Researchers looked at appendage length in subterranean South American rodents. These rodents are not exposed to the surface temperatures much, if at all. Accordingly, the rodents did not experience the longitudinal appendage length variation that Allen’s Rule predicts. However, they did experience variation among burrows at different altitudes – even though the authors suggest that “all species maintain similar temperature conditions within their burrows independently of latitude and altitude”. Factors unrelated to temperature and thermoregulation may be needed to explain these results.

All in all, however, questioning Allen’s Rule is cause for celebration. Simplified biological trends are hardly ever as interesting as the complexity of the real thing. 


Literature Referenced: 

Bidau, Claudio J., Martí, Dardo A., Medina, Alonso I. A test of Allen's rule in subterranean mammals: the genus Ctenomys (Caviomorpha, Ctenomyidae). Mammalia: International Journal of the Systematics, Biology & Ecology of Mammals, Vol. 75 Issue 4 (2011). 

Serrat MA; King D; Lovejoy CO. Temperature regulates limb length in homeotherms by directly modulating cartilage growth. Proceedings Of The National Academy Of Sciences, Vol. 105, No. 49 (2008 Dec 9). 

Nudds, R. L., S. A. Oswald. An Interspecific Test of Allen's Rule: Evolutionary Implications for Endothermic Species. Evolution, Vol. 61, Issue 12 (Dec., 2007). 

Stevenson, R. D. Allen's Rule in North American rabbits (Sylvilagus) and hares (Lepus) is an exception, not a rule. Journal of Mammalogy, Vol. 67, No. 2 (May, 1986). 

Symonds, M., Tattersall, G. Geographical variation in bill size across bird species provides evidence for Allen’s Rule. The American Naturalist, Vol. 176, No. 2. (August 2010). 

Monday, March 12, 2012

"Walking with Tetrapods"

Here is an interesting video from Nature about a radical discovery of fossil footprints that are believed to predate the time at which the current fossil evidence suggests was the first walking-on-land event.


One of the elusive questions in evolutionary biology is when did animals first walk on land? Before this discovery, the fossil record (which includes body fossils and preserved trackways) of the earliest tetrapods dated the water-to-land transition at the Late Devonian period. However, the tetrapod tracks found in Poland suggest that animals first walked the land in the early Middle Devonian period - nearly 20 million years earlier than paleontologists had believed based on early tetrapod body fossils. 

The stride length, relative spacing of the footprints, and absence of body drag (which would be seen if the creature was more fish-like), demonstrate that the pathway was made by tetrapod locomotion.

 The fossilized footprint has clear impressions of short, triangular toes.


As explained in the video, the trackways clearly belong to a four-legged creature - there are obvious imprints of a walking pattern indicative of an animal with forelimbs and hind limbs. Also, a closer look at the individual footprints reveals the distinct impressions of separate digits (toes) and footpads. This novel discovery is now forcing scientists to reevaluate the timeline of the fish-tetrapod transition and reassess the conditions under which our very distant ancestors moved out of the water and onto land, taking their first steps.

Sources:
1. Niedźwiedzki G, Szrek P, Narkiewicz K, Narkiewicz M, Ahlberg PE. Tetrapod trackways from the early Middle Devonian period of Poland. Nature. 2010;463(7277):43–48.

Wednesday, March 7, 2012

Appendix (adding onto Sanjula's previous post)

I found an article on Scientific American about possible functions of the appendix, one of the vestigial organs considered to be relatively useless, in the human body. It states that the fetal appendix actually produces endocrine cells that secrete amines and peptide hormones that are crucial for their development, especially from 11th week of development till young adulthood.

Speaking of fetus and conformational and functional changes, hemoglobins also go through some significant changes from fetal stages to adulthood. There are different types of hemoglobins expressed in the fetus that gets unexpressed.

Fig 1. This shows the different types of hemoglobin genes expressed during fetal stages and adult life.

Fig 2. Schematic representation of hemoglobin gene regulations.


All in all, sometimes it is important to view the functions and purposes of organs and appendages from an evolutionary developmental perspective, also known as evo-devo.


Sources:

1. http://www.scientificamerican.com/article.cfm?id=what-is-the-function-of-t
2. http://sickle.bwh.harvard.edu/hbsynthesis.html

Evolution of Fingers


Which appendages actually differentiate humans from other species? If I were to choose top three most important body parts of the human body, I would definitely choose fingers as one of them. We all know that fingers immensely widen our ability to use tools, perform acute tasks, communicate through sign and body languages, and many more functionalities. Not only do they aid us in daily activities, but also gives us enormous advantages in terms of minuscule and detailed controls.

The most fundamental question that we can ask ourselves is this: why are fingers important, except for the function of making others mad by flipping them? Human fingers are designed so that they are ideal for throwing and clubbing positions. Evolutionarily, the action of “throwing” and “gripping” would have been very important for survival, whether they were throwing spears or rocks, clubbing with wooden sticks, or even holding tools firmly when crafting weapons. These unique features can be found in baseball: pitchers can throw round balls, and batters can swing their bats, all due to our finger structures.



Fig 1. Standard baseball grip.




As we can see from the picture, we would not be able to throw a baseball without our firm thumb. These two types of grips, holding spherical objects or grabbing sticks, make us humans.


Fig 2. Different grips that human fingers can perform.




So do any other species have fingers? Yes! One of the prime examples of organisms with clear finger structures is chimpanzee. The biggest difference between chimpanzee fingers and human fingers is the length: while human thumbs are much longer, the other four fingers of chimpanzees are much longer. The chimpanzees’ elongated metacarpal bones can be explained by their knuckle-walking, where they require much more robust fingers.




Fig. 3 The hand located on the left side represents chimpanzee's hand; on the right represents human's hand. As we can clearly see, chimpanzees have longer fingers except for the thumb. 




These longer fingers help the function of “hook grips” as well, used when grabbing onto tree branches. The human thumb, however, is used when performing a “forward grip,” which is similar to when holding a kendo sword in front of you. Without a sturdy thumb, this motion would be impossible; without this motion, using weaponry that requires swinging clubs would be impossible.



Fig 4. Our friendly Pokemon #151, Mewtwo, with three fingers.


Why do we have five fingers? Why not three like our Pokemon friend, Mewtwo? No one really knows. The scientific term for having five digits is called pentadactyly. In nature, we can seldom find polydactyly, meaning more than 5 digits. The most popular example is the panda:

Fig 5. Panda's palm structure with 6 fingers.




Presently, no one can assuredly explain this natural phenomenon. Does having more digits create better support system for bigger, heavier animals, like panda? Does it spread out partial pressures along each finger? Does more fingers mean more claws to attack the prey? No one knows.

To sum up, fingers and dexterity give human beings the uniqueness and advantage. Let’s be grateful for our fingers, and stop using them for this:

Jk. 




Sources used:

1. Young, W. Richard. "Evolution of Human Hand: the Role of Throwing and Clubbing." Journal of Anatomy, 24 Jan 2003 <http://onlinelibrary.wiley.com/doi/10.1046/j.1469-7580.2003.00144.x/full>
2. "Fish with Fingers." Houston PBS. Evolution Home 2011 <http://www.pbs.org/wgbh/evolution/library/03/4/l_034_03.html>

Some blogs:
3. http://fishfeet2007.blogspot.com/2007/05/life-on-land-evolution-of-five-fingers.html
4. http://www.athro.com/evo/pthumb.html

Sunday, March 4, 2012

Evolution of Pectoral Flippers

     One of the fundamental questions in developmental and evolutionary biology concerns the origin of novel structures. Do appendages evolve de novo (from the beginning) or from pre-existing structures? The adaptive evolution of terrestrial vertebrates to aquatic environments has been well documented through the fossil record. The transition from life on land to life in the ocean was made possible by the appearance of new types of limbs, often accompanied by the loss of certain appendages.
     Here, I examine the origin and diversification of appendages in cetaceans from a terrestrial ancestor. Cetaceans are marine mammals descended from land mammals. This will be a general overview of the evolution of the pectoral fins (commonly known as flippers) in dolphins. The pectoral fins are the forelimbs that allow dolphins to maneuver their locomotion, mainly through steering.
     As revealed in the diagrams above, pectoral fins have a skeletal structure that resembles that of the human forelimb (i.e., the arm, wrist, and hand). Like land mammal forelimbs, flippers have a humerus, radius, ulna, phalanges, and a ball and socket joint. This internal bone structure of the dolphin forelimb is one of the strong indicators that cetaceans arose from a terrestrial ancestor that had frontal appendages that aided in forward movement. However, the skeletal elements of the flipper are foreshortened and modified. Pectoral fins are made of cartilage and bone, but unlike the human forelimb, they are very rigid and stuff, preventing movement at the elbow joint.
     Another interesting deviation from the land mammal forelimb bone structure is that dolphins display hyperphalangy, in which the number of phalanges (finger bones) in the forelimb is greatly increased from the standard number of 3 phalanges per finger. Cetaceans are the only mammals in evolutionary history to undergo hyperphalangy. It is important to note that 3 phalanges per finger do development during embryonic development in dolphins. However, this process does not cease in late embryonic development as in land mammals; the process persists into the fetal period until 9-13 phalanges develop in some fingers.
     Currently, there is a debate over which ancient group of animals gave rise to the cetacean lineage. One theory suggests that cetaceans are descendants of the mesonychid, a terrestrial dog-like animal that lived 55-95 million years ago. However, more recent analyses based on genetics and molecular studies provide more support for the 2nd major theory, which proposes that cetaceans share a common ancestor with the modern-day hippopotamus.
     Based on fossil evidence, the mesonychid was a terrestrial animal that had front and hind limbs containing bones to support its heavy body weight, as well as hoofed toes. Scientists believe that it went into the water to seek food, and over millions of years, it become more adapted to life in the water, leading to the structural changes in its forelimbs. The prevailing theory is that their hind legs reduced in size until disappearing altogether. Thus, the pectoral fins in present-day dolphins and whales are remnants of its ancestor’s life on land, contributing to the aerodynamic shape that allows for efficient swimming.

     Of course, it is impossible to determine the exact evolutionary steps from the “missing link” ancestor to today’s cetaceans, but more recent fossil evidence has bolstered support for the alternative theory that dolphins share a common ancestor with hippos. Many scientists believe that the recent astonishing discovery of a bottlenose dolphin with an extra set of flippers is living proof of the theory. The abnormal bottlenose dolphin is the first of its kind ever to be found, possessing an additional pair of stubby fins near its tail. According to evolutionary biologists, the extra fins are an example of an “atavistic trait,” a genetic trait that appears to be an evolutionary throwback to the ancestral land-dwelling days. For some unknown reason, the dolphin’s extra fins may be the remnants of a pair of hind legs, adding to the theory that dolphins descended from terrestrial four-footed mammals.


Sources:
1. Lovett, Richard. “Dolphin With Four Fins May Prove Terrestrial Origins.” National Geographic News. 8 Nov. 2006. National Geographic Society. <http://news.nationalgeographic.com/news/2006/11/061106-dolphin-legs.html>
2. Dolphin Research Center. 2007. Dolphin Research Center. 1 March 2012. <http://www.dolphins.org/marineed_physiology.php>
3. SeaWorld Animals. 2011. SeaWorld Inc. 1 March 2012. <http://www.seaworld.org/animal-info/info-books/bottlenose/physical-characteristics.htm>
4. American Museum of Natural History. "Getting A Leg Up On Whale And Dolphin Evolution: New Comprehensive Analysis Sheds Light On The Origin Of Cetaceans." ScienceDaily. 24 Sep. 2009. <http://www.sciencedaily.com/releases/2009/09/090924185533.htm>