WEBVTT 00:00:08.842 --> 00:00:12.412 Billions of years ago on the young planet Earth 00:00:12.412 --> 00:00:17.432 simple organic compounds assembled into more complex coalitions 00:00:17.432 --> 00:00:20.122 that could grow and reproduce. 00:00:20.122 --> 00:00:23.372 They were the very first life on Earth, 00:00:23.372 --> 00:00:27.512 and they gave rise to every one of the billions of species 00:00:27.512 --> 00:00:30.602 that have inhabited our planet since. NOTE Paragraph 00:00:30.602 --> 00:00:33.712 At the time, Earth was almost completely devoid 00:00:33.712 --> 00:00:38.282 of what we’d recognize as a suitable environment for living things. 00:00:38.282 --> 00:00:41.725 The young planet had widespread volcanic activity 00:00:41.725 --> 00:00:45.335 and an atmosphere that created hostile conditions. 00:00:45.335 --> 00:00:48.835 So where on Earth could life begin? NOTE Paragraph 00:00:48.835 --> 00:00:51.365 To begin the search for the cradle of life, 00:00:51.365 --> 00:00:56.715 it’s important to first understand the basic necessities for any life form. 00:00:56.715 --> 00:01:01.940 Elements and compounds essential to life include hydrogen, methane, nitrogen, 00:01:01.940 --> 00:01:05.468 carbon dioxide, phosphates, and ammonia. 00:01:05.468 --> 00:01:09.828 In order for these ingredients to comingle and react with each other, 00:01:09.828 --> 00:01:13.260 they need a liquid solvent: water. 00:01:13.260 --> 00:01:15.700 And in order to grow and reproduce, 00:01:15.700 --> 00:01:19.260 all life needs a source of energy. 00:01:19.260 --> 00:01:22.300 Life forms are divided into two camps: 00:01:22.300 --> 00:01:26.030 autotrophs, like plants, that generate their own energy, 00:01:26.030 --> 00:01:31.330 and heterotrophs, like animals, that consume other organisms for energy. 00:01:31.330 --> 00:01:35.920 The first life form wouldn’t have had other organisms to consume, of course, 00:01:35.920 --> 00:01:38.391 so it must have been an autotroph, 00:01:38.391 --> 00:01:43.241 generating energy either from the sun or from chemical gradients. NOTE Paragraph 00:01:43.241 --> 00:01:46.641 So what locations meet these criteria? 00:01:46.641 --> 00:01:49.751 Places on land or close to the surface of the ocean 00:01:49.751 --> 00:01:52.591 have the advantage of access to sunlight. 00:01:52.591 --> 00:01:57.631 But at the time when life began, the UV radiation on Earth’s surface 00:01:57.631 --> 00:02:01.221 was likely too harsh for life to survive there. 00:02:01.221 --> 00:02:04.511 One setting offers protection from this radiation 00:02:04.511 --> 00:02:07.221 and an alternative energy source: 00:02:07.221 --> 00:02:11.321 the hydrothermal vents that wind across the ocean floor, 00:02:11.321 --> 00:02:16.847 covered by kilometers of seawater and bathed in complete darkness. NOTE Paragraph 00:02:16.847 --> 00:02:20.771 A hydrothermal vent is a fissure in the Earth’s crust 00:02:20.771 --> 00:02:23.891 where seawater seeps into magma chambers 00:02:23.891 --> 00:02:27.291 and is ejected back out at high temperatures, 00:02:27.291 --> 00:02:32.521 along with a rich slurry of minerals and simple chemical compounds. 00:02:32.521 --> 00:02:35.276 Energy is particularly concentrated 00:02:35.276 --> 00:02:39.566 at the steep chemical gradients of hydrothermal vents. NOTE Paragraph 00:02:39.566 --> 00:02:42.882 There’s another line of evidence that points to hydrothermal vents: 00:02:42.882 --> 00:02:48.952 the Last Universal Common Ancestor of life, or LUCA for short. 00:02:48.952 --> 00:02:54.042 LUCA wasn’t the first life form, but it’s as far back as we can trace. 00:02:54.042 --> 00:02:57.959 Even so, we don’t actually know what LUCA looked like— 00:02:57.959 --> 00:03:02.029 there’s no LUCA fossil, no modern-day LUCA still around— 00:03:02.029 --> 00:03:07.159 instead, scientists identified genes that are commonly found in species 00:03:07.161 --> 00:03:11.161 across all three domains of life that exist today. 00:03:11.161 --> 00:03:15.501 Since these genes are shared across species and domains, 00:03:15.501 --> 00:03:19.488 they must have been inherited from a common ancestor. 00:03:19.488 --> 00:03:25.298 These shared genes tell us that LUCA lived in a hot, oxygen-free place 00:03:25.298 --> 00:03:28.488 and harvested energy from a chemical gradient— 00:03:28.488 --> 00:03:31.748 like the ones at hydrothermal vents. NOTE Paragraph 00:03:31.748 --> 00:03:34.718 There are two kinds of hydrothermal vent: 00:03:34.718 --> 00:03:37.398 black smokers and white smokers. 00:03:37.398 --> 00:03:41.488 Black smokers release acidic, carbon-dioxide-rich water, 00:03:41.488 --> 00:03:47.268 heated to hundreds of degrees Celsius and packed with sulphur, iron, copper, 00:03:47.268 --> 00:03:50.136 and other metals essential to life. 00:03:50.136 --> 00:03:55.196 But scientists now believe that black smokers were too hot for LUCA— 00:03:55.196 --> 00:04:00.163 so now the top candidates for the cradle of life are white smokers. NOTE Paragraph 00:04:00.163 --> 00:04:01.637 Among the white smokers, 00:04:01.637 --> 00:04:07.067 a field of hydrothermal vents on the Mid-Atlantic Ridge called Lost City 00:04:07.067 --> 00:04:11.157 has become the most favored candidate for the cradle of life. 00:04:11.157 --> 00:04:16.607 The seawater expelled here is highly alkaline and lacks carbon dioxide, 00:04:16.607 --> 00:04:21.101 but is rich in methane and offers more hospitable temperatures. 00:04:21.101 --> 00:04:25.770 Adjacent black smokers may have contributed the carbon dioxide necessary 00:04:25.770 --> 00:04:28.122 for life to evolve at Lost City, 00:04:28.122 --> 00:04:31.652 giving it all the components to support the first organisms 00:04:31.652 --> 00:04:36.132 that radiated into the incredible diversity of life on Earth today.