Resurrected Seeds: The Evolution of Dormancy
For millions of years, the first plants on Earth had a form of reproduction in which an independent gametophyte (the haploid, sexual stage of the plant lifecycle) produced eggs and motile sperm. When it rained, or when these gametophytes grew close to other sources of free water, the sperm would swim from the from the male organ of one plant to the female organ of another, whereupon fertilization would occur. Several plant lineages still reproduce this way, including bryophytes, lycophytes, and ferns. The obvious downside of this method, however, is the reliance on water to give rise to the next generation of plants. While several species can circumvent this problem by reproducing asexually (which comes with its own downsides), the dependence on water in many of Earth’s earliest plants meant that they were restricted to moist or swampy areas, leaving much of the planet uncolonized.
But around 380 million years ago, evolution took a giant leap, with the advent of the first seeds, which arose in an enigmatic group of plants called ‘seed ferns’ (Linkies et al., 2010). These plants were trees and had seed-like structures, but their leaves looked exactly like that of modern ferns, fiddleheads and all. Seed ferns have long since gone extinct, and it’s currently unknown how they’re related to modern groups of plants, but the evolution of seeds in this group would completely alter Earth’s ecosystems and eventually allow for the evolution of mammals. Just how these seeds evolved is a fascinating story in its own right (and one that I’m saving for my next article on seed ferns), but one of the most significant innovations was the retention of sperm in pollen. The sperm of seed plants aren’t motile, like those of other land plants; rather, the pollen will grow tubes through a structure called the micropyle, transporting the sperm directly to the eggs within. Without the need for water to reproduce, plants were able to colonize drier portions of the planet that had until then remained uninhabitable.
Another important innovation was the evolution of dormancy. Just because these plants didn’t need water for fertilization doesn’t mean their seeds didn’t need water to support the developing embryo. It doesn’t appear that any of the seed ferns were able to produce dormant seeds, but a fossil from a 300 million-year-old Walchian conifer shows a perfectly preserved embryo, indicating the seed was likely in a quiescent stage at the time of fossilization (Mapes, Rothwell, and Haworth, 1989). Now seeds had the ability to germinate only when conditions were favorable to the growth of a new plant. Some seeds can even remain dormant for several thousand years, as was recently shown to be the case in two separate studies. The first will take us back to Judea during the time of Roman occupation and civil unrest, and the second will take us back further still, to the ice ages of the Pleistocene.
THE SIEGE OF MASADA
In the 1st century B.C.E. (before common era), the kingdom of Judea was conquered by the Roman Empire, and for the next 100 years was ruled by a series of Roman governors, some worse than others. The very worst carried out brutal killing sprees upon even the smallest provocation, prompting the Jewish citizens to revolt against Roman rule in 66 C.E. While the Jews saw mild success at first, the population of its capital city, Jerusalem, was woefully divided between those who wanted to make peace with Rome and the hardcore zealots who would stop at nothing to gain independence, even slaughtering their own people with opposing views, which they did in the thousands. The resulting force was no match for the Roman army, which burned Jerusalem to the ground in 70 C.E.
There remained three well-fortified holdouts outside of the main city, however, and in 71 C.E., the Roman general Lucilius Bassus began picking them off one by one. The very last to hold out was the seemingly impregnable fortress of Masada, built by King Herod atop honey-colored cliffs with sheer drops on all sides. A small, winding path was the only entrance to the fortress, which would never have supported the large Roman war machines. So they built a ramp up the mountainside, using slave labor, many of them Jews who had been enslaved in Jerusalem and surrounding cities. After nine months, the ramp was complete, and the Romans moved their battering ram up to the outer wall, which they breached in short order. The 600 defenders inside retreated to the inner court, but it was clear that there was little hope for survival. Their leader, Eleazar Ben Yair, who had escaped from the destruction in Jerusalem three years earlier, convinced his men to form a suicide pact. After killing their wives and children, the soldiers took their own lives. When the Romans broke through the second wall, the only sound to greet them was that of their own war cry echoing off the chamber walls. The only survivors were two women and five children, who had hidden in a cistern during the killings.
THE SEEDS OF MASADA
While the siege of Masada remains a tragic event in Jewish history, archeologists at the site between 1963 and 1965 uncovered life in the storerooms of the ancient fortress. Part of what made Masada such a great stronghold were the deep cisterns that could hold a massive amount of rainwater, as well as the dry, desert air, which meant that food could be stored for long periods of time without spoiling. It was this latter quality that lead to the discovery of the seeds of an extinct date palm that had been hidden away since the time of Roman occupation, either just before or after the siege (the age of several seeds was verified using radiocarbon dating) (Sallon et al., 2008).
The seeds were given to paleobotanist Elaine Solowey at Bar-Ilan University in Tel-Aviv, who promptly put them in her desk drawer and out of mind while she read up on methods of revitalizing old seeds. In 2005, on the day of the Jewish holiday Tu BiShvat, during which it is a custom to plant trees, Solowey planted the ancient Masada seed and waited to see the outcome. Within two months the seed had germinated, and now, 12 years later, the pollen from the tree has been used to fertilize a female in a closely related group.
AN ANCIENT CACHE OF SQUIRREL NUTS
While 2,000 years is certainly an incredibly long amount of time for seeds to remain dormant, there are trees that are even older, such as the bristlecone pine given the moniker ‘Methuselah,’ which is reportedly some 4,848 years old. But it was recently shown that seeds have the potential to lie in what for much longer periods of time than previously expected. A team of Russian scientists recently uncovered the seeds of an ancient flower buried beneath 125 feet of loess ice, which had been frozen at -7 °C for 32,000 years (samples were radiocarbon dated) (Yashina et al., 2012). These seeds, found amid the frozen remains of mammoths and woolly rhinoceroses, had been stored in a burrow by an industrious squirrel who promptly forgot where they were buried. Researchers identified the seeds as belonging to the narrow-leafed campion (Silene stenophylla). Initial attempts by researchers to germinate the seeds were unsuccessful, but upon removal of portions of the placenta (the portion of the seed that provides nutrients to the developing embryo), the team was able to grow new plants from the ancient stock, making them the oldest plants on Earth.
Humans have recently begun to exploit the regenerative properties of seed dormancy. The resurrection of the narrow-leafed campion after thousands of years of quiescence demonstrated that cold temperatures can be utilized to preserve endangered plants for the future. The Svalbard Global Seed Vault in Norway was built back in 2008 in an attempt to do just that. Operating under the premise that our current diversity of food crops should be preserved to safeguard against global catastrophes, researchers from around the world have donated seeds. The vault was only ever meant to be opened in the face of dire need, so it’s somewhat discomfiting that an extraction was already made just two years ago, not due to the cataclysmic onset of floods, droughts, or earthquakes, but rather because of an entirely man-made disaster: war. Researchers working for the International Center for Agricultural Research in Dry Areas in the city of Aleppo, Syria, had to evacuate their facility due to the civil war that continues to rage in that country, leaving behind some 135,000 crop varieties specifically bred to bring life to the desert. The researchers have since relocated to the cities of Lebanon and Morocco where the seeds they extracted from Svalbard are flourishing once again.
Linkies, A., Graeber, K., Knight, C., & Leubner‐Metzger, G. (2010). The evolution of seeds. New Phytologist 186: 817-831.
Mapes, G., Rothwell, G. W., & Haworth, M. T. (1989). Evolution of seed dormancy. Nature 337: 645-646.
Sallon, S., Solowey, E., Cohen, Y., Korchinsky, R., Egli, M., Woodhatch, I., Simchoni, O., & Kislev, M. (2008). Germination, genetics, and growth of an ancient date seed. Science 320: 1464-1464.
Yashina, S., Gubin, S., Maksimovich, S., Yashina, A., Gakhova, E., & Gilichinsky, D. (2012). Regeneration of whole fertile plants from 30,000-y-old fruit tissue buried in Siberian permafrost. Proceedings of the National Academy of Sciences 109:4008-4013.
Raham, G. (2012, November 1). Resurrection from permafrost and other ice-age legacies. North Forty News. Retrieved from http://www.northfortynews.com/resurrection-from-permafrost-and-other-ice-age-legacies/
Seward, D. (2009). Jerusalem’s Traitor: Josephus, Masada, and the Fall of Judea. Philadelphia, PA: De Capo Press.
Taylor, T. N., Krings, M., & Taylor, E. L. (2009). Paleobotany: the biology and evolution of fossil plants. London: Elsevier.