OFFICIAL51 What is the lecture mainly about?

[00:02.00]NARRATOR: Listen to part of a lecture in a botany class.[00:06.72]FEMALE PROFESSOR: So, continuing with crop domestication, and corn—or, um, maize, as it's often called. Obviously it's one of the world's most important crops today. [00:17.06] It's such a big part of the diet in so many countries, and it's got so many different uses, that it's hard to imagine a world without it. [00:24.91]But because it doesn't grow naturally, without human cultivation, and because there's no obvious wild relative of maize… uh, well, for the longest time, researchers weren’t able to find any clear link between maize and other living plants. [00:39.79] And that's made it hard for them to trace the history of maize.
[00:43.49]Now, scientific theories about the origins of maize first started coming out in the 1930s. One involved a plant called teosinte.[00:53.75]Teosinte is a tall grass that grows wild in certain parts of Mexico and Guatemala. [00:59.35]When researchers first started looking at wild teosinte plants, they thought there was a chance that the two plants—um, maize and teosinte—were related. [01:09.29] The young wild teosinte plant looks a lot like the corn plant, [01:13.73]and the plants continue to resemble each other—at least superficially—even when they're developed.
[01:19.79] But when the scientists examined the fruits of the two plants, it was a different story. [01:25.15]When you look at ripe corn, you see row upon row of juicy kernels… um, all those tiny little yellow squares that people eat. [01:34.65] Fully grown teosinte, on the other hand, has a skinny stalk that holds only a dozen or so kernels behind a hard, um, almost stonelike casing. [01:45.60] In fact, based on the appearance of its fruit, teosinte was initially considered to be a closer relative to rice than to maize.
[01:54.95]But there was one geneticist, named George Beadle, who didn't give up so easily on the idea that teosinte might be… well… the “parent” of corn.[02:06.33]While still a student in the 1930s, Beadle actually found that the two plants had very similar chromosomes—very similar genetic information. [02:15.46]In fact, he was even able to make fertile hybrids between the two plants. [02:20.02]In hybridization, you remember, the genes of two species of plants are mixed to produce a new, third plant—a hybrid. [02:27.84]And if this offspring—this hybrid—is fertile, then that suggests that the two species are closely related genetically. [02:35.33]This new, hybrid plant looked like an intermediate, right between maize and teosinte. [02:41.03] So, Beadle concluded that maize must've been developed over many years, uh, that it is a domesticated form of teosinte.[02:50.83]Many experts in the scientific community, however, remained unconvinced by his conclusions. [02:56.88]They believed that, with so many apparent differences between the two plants, it would have been unlikely that ancient—that prehistoric peoples could’ve domesticated maize from teosinte. [03:08.64] I mean, when you think about it, these people lived in small groups, and they had to be on the move constantly as the seasons changed. [03:16.73]So for them to selectively breed, to have the patience to be able to pick out just the right plants… and gradually—over generations—separate out the durable, nutritious maize plant from the brittle teosinte that easily broke apart… it's a pretty impressive feat,
[03:34.60]and you can easily see why so many experts would have been skeptical.[03:38.28]But, as it turns out, Beadle found even more evidence for his theory when he continued his experiments, producing new hybrids, to investigate the genetic relationship between teosinte and maize. [03:50.62]Through these successive experiments, he calculated that only about five specific genes were responsible for the main differences between teosinte and maize—[04:01.15]the plants were otherwise surprisingly similar genetically.
[04:04.70]And more recently, botanists have used modern DNA testing to scan plant samples collected from throughout the Western Hemisphere. [04:12.98]This has allowed them to pinpoint where the domestication of maize most likely took place—[04:18.55] and their research took them to a particular river valley in southern Mexico. [04:23.00]They've also been able to estimate that the domestication of maize most likely occurred about 9,000 years ago. [04:29.98]And subsequent archaeological digs have confirmed this estimate. [04:34.20]In one site, archaeologists uncovered a set of tools that were nearly 9,000 years old. [04:40.22]And these tools were covered with a dusty residue… a residue of maize, as it turns out…[04:46.34]thus making them the oldest physical evidence of maize that we've found so far.