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North and South America are the natural habitat of cactus plants, which grow well in desert environments. Although humans have brought several cactus species to other continents relatively recently, there is one species, the wicker cactus, that has grown in the wild outside of the Americas for a much longer time. The wicker cactus has grown on both sides of the Atlantic Ocean, in the Americas and Africa, for so long that no one knows for certain how this normally American plant arrived in Africa. Several theories have been proposed.



Birds

The first theory is that the wicker cactus was spread like many other plants: by birds. Its seeds could have been eaten by a migrating bird, which stored them in its digestive tract and then passed them on as waste after crossing to another continent. In a suitable environment on the new continent, the plants could have developed from these transported seeds and then reproduced. This theory is supported by the fact that many bird species are known to make regular migrations of great distances.



Predates separation of continents

A second theory is that the wicker cactus has existed for so long that it is actually older than the ocean that now separates Africa and South America. Long ago, Africa and South America were part of one giant continent. In this tract and then passed them on as waste after crossing to another continent. In a suitable environment on the new continent, the plants could have developed from these transported seeds and then reproduced. This theory is supported by the fact that many bird species are known to make regular migrations of great distances.



Predates separation of continents

A second theory is that the wicker cactus has existed for so long that it is actually older than the ocean that now separates Africa and South America. Long ago, Africa and South America were part of one giant continent. In this explanation, the wicker cactus first evolved during the ancient time before the continents separated. As the two land masses slowly drifted apart over millions of years, early wicker cactus plants were left on each side of the ocean.



Sailors

The third theory is that sixteenth-century sailors carried the wicker cactus from South America to Africa on their boats. Records show that when Portuguese and Spanish sailors traveled to the Americas, they frequently brought plants back and forth across the ocean. Since the wicker cactus can survive for a long time without soil, it could have easily lived through such a trip.

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None of the theories presented in the reading provides a satisfactory explanation for how the wicker cactus spread across the ocean, first, about birds spreading seeds. Well, some birds do eat the seeds of the wicker cactus. The Cactus produces small, white fruits with the seeds inside them, and some species of birds eat the fruit and spread its seeds through their digestion. The problem is that none of these bird species typically fly directly across the Atlantic Ocean from the Americas to Africa. There are other birds that cross the Atlantic Ocean and travel from the Americas to Africa regularly, but those birds mainly eat insects and fish. They typically don't feed on fruit, so they probably would not consume wicker cactus seeds. Second, the idea that the wicker cactus evolved before the continent separated has been called into question. You see, until recently, no one knew exactly when cactus species first evolved, but researchers can now study the DNA or genes of a species to estimate how long this species has existed, and new DNA studies suggest that no cactus existed when South America and Africa were connected in one giant continent, based on the DNA studies, scientists now believe that all cactus species, including the wicker cactus, first evolved much later, millions of years after the separation of continents. Third the sailor theory, if sailors had carried the wicker cactus to Africa on boats, we would expect to find it growing mostly in places where ships would have arrived near ports. But the wicker cactus usually grows in parts of Africa that are far away from ports and not in any of Africa's port cities. What's more, some of the places where the cactus is found are very far inland, deep in the middle of the African continent, in areas ships could never even approach in.
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Summarize the points made in the lecture, being sure to explain how they challenge the specific points made in the reading passage.

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