A small strip of land between the Atlantic and Pacific ocean might seem like an odd place to look for answers about evolution. But, the Isthmus of Panama is considered to be one of the most important pieces of evidence in questions that involve the triggering of the last Ice Age, the separation of marine species, global ocean current circulation, and even the land connection between North and South America (O’Dea et al. 2016). The one problem is: scientists can’t agree on when the isthmus formed.
The isthmus, a narrow strip of land connecting two landmasses, was thought to have formed and closed around three million years ago (Ma), based on converging geological and biological evidence (Stehli and Webb 1985, chaps. 2, 4, 5, 12; Keigwin 1978). Subduction of the Pacific Plate beneath the Caribbean Plate formed the volcanic Panama arc, which eventually then collided with South America. This helped close the Central American Seaway (Stehli and Webb 1985, chaps. 2-4). Around this time, the Great American Biotic Interchange, a massive land mammal migration, took place, as well as various fossils (Foraminifera) began to show evidence of speciation (Stehli and Webb 1985, chaps. 5, 12; Keigwin 1978). These observations supported the 3 Ma closure date, but they did not provide an explanation of how the closure happened.
By the 1990s, advances in molecular techniques allowed scientists to investigate whether the isthmus had formed gradually rather than all at once. One approach was to study marine species on opposite sides of the isthmus. The formation of the emerging land bridge isolated populations that had shared the same seaway, allowing allopatric speciation (Scott 1916). By comparing accumulated genetic differences, scientists could estimate when these populations diverged. In 1993, researchers compared seven pairs of closely related snapping shrimp (Alpheus) from opposite sides of the isthmus to estimate when their populations diverged (Table 1) (Knowlton et al. 1993). They found that some populations had stopped exchanging genes millions of years before the previously accepted 3 Ma closure date. In Table 1, the taxa column identifies the populations being compared and the bolded rows indicate the seven pairs identified as closest relatives. Genetic divergence is measured using alloenzyme differences and mitochondrial DNA (mtDNA) sequencing, with higher values indicating greater genetic divergence (Knowlton et al. 1993).
Table 1. Genetic divergence between snapping shrimp taxa from the Pacific (P) and Caribbean (C) side of the Isthmus of Panama (Knowlton et al. 1993).
This evidence was one of several findings that challenged the original theory that the Isthmus of Panama closed relatively suddenly around 3 Ma (Bacon et al. 2015; Coates et al. 2013). Another clue came from uranium-lead dating technology, which was not available previously. Researchers found minerals called zircons originating from Panama in sedimentary rocks dating back 13 to 15 Ma (Figure 1). Since these minerals could have been transported there by rivers flowing from the Panama arc, this means that parts of the isthmus had already emerged above sea level and had become connected to South America much earlier than previously thought (Montes et al. 2015).
So, are Panama’s shrimp telling time correctly? They’re not necessarily wrong, but each piece of evidence reveals a different part of the story. Earlier research placed the isthmus’ complete closure around 3 Ma (Stehli and Webb 1985; Keigwin 1978). However, later studies proposed that marine populations speciated much before and that parts of the isthmus had emerged as early as 13-15 Ma (Knowlton et al. 1993; Montes et al. 2015). Ultimately, although the 3 Ma date still holds, it is no longer the complete formation story of the Isthmus of Panama.
References
Bacon, Christine D., Daniele Silvestro, Carlos Jaramillo, Brian Tilston Smith, Prosanta Chakrabarty, and Alexandre Antonelli. “Biological Evidence Supports an Early and Complex Emergence of the Isthmus of Panama.” Proceedings of the National Academy of Sciences 112, no. 19 (2015): 6110–15. https://doi.org/10.1073/pnas.1423853112.
Coates, Anthony G., and Robert F. Stallard. “How Old Is the Isthmus of Panama?” Bulletin of Marine Science 89, no. 4 (2013): 801–13. https://doi.org/https://doi.org/10.5343/bms.2012.1076.
Knowlton, Nancy, Lee A. Weight, Luis Aníbal Solórzano, DeEtta K. Mills, and Eldredge Bermingham. “Divergence in Proteins, Mitochondrial DNA, and Reproductive Compatibility Across the Isthmus of Panama.” Science 260, no. 5114 (1993): 1629–32. http://www.jstor.org/stable/2881712.
Keigwin, Lloyd D. “Pliocene Closing of the Isthmus of Panama, Based on Biostratigraphic Evidence from Nearby Pacific Ocean and Caribbean Sea Cores.” Geology 6, no. 10 (1978): 630. https://doi.org/10.1130/0091-7613(1978)6%3C630:PCOTIO%3E2.0.CO;2.
Montes, C., A. Cardona, C. Jaramillo, et al. “Middle Miocene Closure of the Central American Seaway.” Science 348, no. 6231 (2015): 226–29. https://doi.org/10.1126/science.aaa2815.
O’Dea, Aaron, Harilaos A. Lessios, Anthony G. Coates, et al. “Formation of the Isthmus of Panama.” Science Advances 2, no. 8 (2016): e1600883. https://doi.org/10.1126/sciadv.1600883.
Scott, W. B. “The Isthmus of Panama in Its Relation to the Animal Life of North and South America.” Science 43, no. 1100 (1916): 113–24. http://www.jstor.org/stable/1640614.
Stehli, Francis G., and S. David Webb. 1985. The Great American Biotic Interchange. Plenum Press.
Comments
16 Responses to “Are Panama’s Shrimp Telling Time Right?”
Hi iSci, I chose this topic because I thought it was very relevant to the evolution unit we’re doing in LifeSci, especially the part about allopatric speciation. Many marine populations were separated by the formation and complete closing of the isthmus and they’ve evolved in very different ways. Additionally, I connected the plate tectonics material and dating techniques we’re learning about in Earthsci to a relevant scientific debate that researchers are still working on. Hopefully it’s an interesting read!
Hey Mansi,
This is a really well written blog! A couple edits I have:
– Maybe consider using a figure to show the geographical features (plates) that you described in paragraph 2 so your readers can visualize it better
– In table 1, explain what is being show in more depth. Especially in the “taxa” column, it is unclear what it is describing from just the figure and your caption. Also explain what the bolded rows signify.
– In the last sentence of your figure 1 caption, remove the “at least” or rephrase the sentence to make it more concise.
Happy editing!
Hanna
Hi Hanna – thank you so much for your feedback! I included your second and third point and I think it really helped the blog post flow more. Unfortunately, I didn’t have the word count to include another figure but I reworked P2 to make it more clear.
Thanks again!
Mansi
Hi Mansi, this was a very interesting blog that combined both Earth Science and Life Science and I really enjoyed reading it! Here are some suggestions that will hopefully help you in the editing stage:
— For your table and figure captions, try to make them smaller to make them stand out. As well, maybe try to make the”references” word bigger and bold.
– Refer to Figure 1 in your paragraph before showing the figure
– Just be mindful of punctuations, for example, in P1S3, delete et the comma after “problem” for better flow, in P4S2, add a comma after “technology” in P5S5, add a comma after “instead.”
-I agree with Hanna to explain your Table 1 caption further
Cheers,
Jojo
Hey Jojo! Thanks for the feedback. I was able to implement all of your suggestions so thank you for helping me with the clarity/punctuation of the post!
Best,
Mansi
Hi Mansi,
Great job writing your blog post, and such a cool topic!
– P1S3, I recommend adding an em dash between “Unfortunately, there is one problem —scientists can’t necessarily agree on when the isthmus formed” for better flow.
– Remember to add your in-text citations for the first two sentences in paragraph 2; if you used the same citation as in sentence 3, the correct format would be to include it in the first sentence and the last (you don’t have to cite anything in the middle)!
– A few of your in-text citations include a comma between author names and date; Chicago style doesn’t require a comma.
Good luck on your editing process!
– Harshita
Hi Harshita,
Thanks a ton for the feedback! I was able to implement all of your changes so hopefully that helps with the citation formatting and the sentence structure.
Mansi
Hi Mansi,
Great work on your blog post, it was really an interesting read.
Here are a few feedbacks:
– P5S1, I would recommend getting rid of the “right” in the question, so it flows better as a question.
– I would recommend formatting the References as a title to separate it from the rest of the text.
– In your figure 1 caption, I would recommend removing the word “which” from the sentence “Reconstruction of the Panama arc and northwestern South America during the middle Miocene (13 to 15 Ma), which shows the region’s geography and zircon deposits during an earlier stage of the formation.”
– As well as rephrase the sentence, “At least a segment of the Panama arc had emerged meaning the isthmus did not quickly solidify or close 3 Ma (Montes et al., 2015)” to Because a segment of the Panama arc had emerged, the isthmus did not quickly solidify or close 3 Ma.
Other than that, great job on your blog post. Happy editing!
Saanvi
Hi Saanvi,
I’m glad you liked the blog post. I was able to implement all of your suggested changes so thank you for the feedback!
Regards,
Mansi
Hi Mansi,
This was a really interesting post on a topic I had never even thought about before!
A couple of suggestions:
1. I was very confused at the beginning of the post because I did not know what the word “isthmus” meant. A map to show the strip of land that is now Panama, early in the post, would be helpful. If you have the word count to do it, you might also want to add a sentence about how the Panama canal has more recently reconnected the Atlantic and Pacific oceans, but you will be focusing on how they were originally separated.
2. I agree with Harshita that some more in-text citations earlier in P2 would be helpful. The in-text citation in P2S3 lists 4 chapters, which is a bit vague and does not show which parts of information come from which parts of the book. I would put an in-text citation at the end of P2S1 and try to only refer to the chapters you specifically used for that sentence in the in-text citation. Also, the in-text citation says “Stehli and Webb” but the entry in the reference list names three authors, so double check the authors of that book and keep it consistent throughout.
3. At the end of paragraph 3, you explain how genetic differences showed that some shrimp populations had diverged long before 3Ma, but in P5S2 you mention that the shrimp populations began to evolve separately 3Ma, contradicting your previous statement. If you are referring to a different study with different findings about shrimp in P5S2, make sure to cite it, otherwise I would edit paragraph 5 to make sure it matches the data presented in paragraph 3. Don’t forget to add some in-text citations in paragraph 5 as well.
This was a fascinating article! I especially found Figure 1 very helpful to understand the part about how the isthmus could have formed slowly over time.
Happy editing!
Laura
So sorry, I just realized that “Francis G.” is the first name of Stehli, not an additional author. Please ignore the last part of my second suggestion.
Hi Laura,
I appreciate the detailed feedback! Thank you for also pointing out that not everyone would know what the word “isthmus” meant – that totally slipped my mind. I was able to implement almost all of your feedback points except for the bit with the vague chapters regarding Stehli and Webb’s book. I used those all of those chapters for the background knowledge and the sentences so although I tried to split it up in this new version, I had to keep some of the chapters together.
Other than that, the feedback really helped. Thank you again!
Mansi
Hey Mansi,
I really enjoyed reading your article, and I think this is an engaging and interdisciplinary topic! Some suggestions I have for you are:
– As Laura mentioned, you could maybe include a brief description of what an Isthmus is before delving into the details.
– I would break up this sentence, “Subduction of the Pacific Plate under the Caribbean Plate caused an underwater volcanic chain known as the Panama arc, which ended up moving westward with the movement of the South American Plate” into two separate bits to make it easier to read.
– Reword the sentence “The shrimps’ genetic differences were recorded… ” into something like “The genetic divergence between shrimp populations may indicate when they separated from one another.”
– If you have space to add more, you could include a conclusion that may explain why this topic is important and what future steps scientists might take.
Overall, great job, and I look forward to reading your final draft!
– Swetha S
Hey Swetha,
I’m glad you enjoyed the blog and thank you for your feedback! I made changes as per your first three points; however, I wasn’t able to fully make the last change due to the limited word count. I tried to re-word my conclusion in a different way so that it didn’t introduce any new ideas so hopefully this worked a little bit better than the old one!
Best,
Mansi
Hi Mansi,
This was such a fun blog post! It reminded me of Rodrigo.
A few suggestions for your edits:
1. I would suggest consolidating the last two sentences and removing ‘necessarily’ from your last sentence. Something along the lines of ‘The one problem is: scientists can’t agree on when the isthmus formed’.
2, In P2, I would rewrite it to: ‘Additionally, seafloor cores showed that marine fossils (Foraminifera) began speciating on either side of Panama from 3.5 Ma (Keigwin 1978).’
3. Also, since Foraminifera is a phylum or class, I would capitalize the F.
4. I would also remove ‘though’ from P3S1 to improve flow.
5. Also, I would recheck your citation for Nancy Knowlton. As I believe it should just be (Knowlton et al. 1993).
6. For your figure captions, there should be an option when you click your image to insert a caption, which might help differentiate your captions from the main text.
Great blog post! Can’t wait to read the final draft.
Bohmie
Hi and thank you so much for this feedback, Bohmie! I was able to implement all of your suggested changes and I really liked the first point especially as it condensed that section of the blog. I didn’t really liked the way it flowed before so this newer version is definitely more clear.
Thanks again!
Mansi