How Did Multicellularity Evolve? — Transcript

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1. 0:21Your screen is currently filling up with clouds of an algae that resembles a unicellular flagellate
2. 0:27known as Chlamydomonas, though we’re not quite sure that that’s definitely what it
3. 0:32is.
4. 0:33The algae are swimming towards the lit part of the stage, and as they amass, they form
5. 0:38a blanket over the slide that looks like a kind of very festive green television static.
6. 0:49Meanwhile, on the more orderly end of things, we have this Volvox.
7. 0:57It’s much larger than the tiny algae we just saw, and it’s also much more serene.
8. 1:03Just a nice chill green ball in the middle of your screen.
9. 1:07It doesn’t seem to have much in common with the Chalmydomonas.
10. 1:10I mean, yes, they’ve got some green in common.
11. 1:13But that’s not particularly difficult to come by in the microcosmos.
12. 1:18But these two organisms are both part of the same family of algae called volvocine algae.
13. 1:25And when you look at a more extended family portrait, maybe you will start to see the
14. 1:30resemblance.
15. 1:31You’ve got your gonium, looking like several of those little green algae stuck together.
16. 1:37And then there’s this active pandorina, also packing together several green cells
17. 1:42in its colony body as it moves around.
18. 1:45And then we see the Eudorina….and then the pleodorina, which looks increasingly like
19. 1:51the orderly sphere of the Volvox In fact, this combination of footage feels
20. 1:58less like a family portrait and more like a series of yearbook photos taken from kindergarten
21. 2:04through senior year.
22. 2:06Except instead of the striking passage of time documented through the changes in a child’s
23. 2:12face, this is the striking passage of multicellular evolution documented through the changes in
24. 2:19one algal family.
25. 2:22Multicellularity is one of those things that is so entwined with notions of complexity
26. 2:28and the forward marching of evolution that it might feel like it has to be one of those
27. 2:34singular evolutionary events: a moment that occurred once or maybe twice, and was maybe
28. 2:41too difficult to ever replicate again.
29. 2:44But it turns out that making the jump from unicellular to multicellular life has actually
30. 2:50happened quite a few times in our planet’s history.
31. 2:54It’s maybe surprisingly not that difficult a transition.
32. 2:58But finding a way to study it is.
33. 3:05Many of these jumps happened too long ago, and evolution has removed the organisms whose
34. 3:11bodies contain the story of how they or their ancestors went from living on their own to
35. 3:18teaming up with others.
36. 3:19But fortunately, the volvocine algae went about this transition much more recently--likely
37. 3:25around 200 million years ago.
38. 3:28And that leaves us with a gradient of cellular complexity that starts with the unicellular
39. 3:33Chlamydomonas, transitions to the colonial gatherings of cells that make up the gonium
40. 3:39through pleodorina, and then leads us to the multicellular Volvox.
41. 3:44That ordering though is a little misleading in its simplicity.
42. 3:49Evolution isn’t nearly so neat to work in a straight line across a whole family.
43. 3:54But laying out the organisms in this way helps us to understand that steps are involved in
44. 4:01taking the species from unicellular to multicellular.
45. 4:05One of the most apparent differences when we compare those single-celled algae to Volvox
46. 4:11is how much bigger the Volvox is.
47. 4:13The world is made up of large unicellular organisms and small multicellular organisms.
48. 4:19But in this family, there is an increase in size that is connected to how many cells an
49. 4:25individual contains.
50. 4:27So, how to do that?
51. 4:30Well, volvocine algae primarily reproduce asexually.
52. 4:34But where many microbes divide through binary fission--dividing into two daughter cells
53. 4:39that then grow and divide again--most volvocine algae are a little more chaotic.
54. 4:45They divide through multiple fission, which means that any cell that’s going to reproduce
55. 4:51starts by growing...a lot.
56. 4:54If they want two daughter cells, they grow twice as big.
57. 4:57If they want to make four, they grow four times as large.
58. 5:01And once they get as big as they need to, the algae goes through a rapid series of divisions
59. 5:06to produce all those daughter cells in quick succession.
60. 5:10For Chlamydomonas, you might get around 4 - 8 daughter cells.
61. 5:14A gonium colony might have 8 - 16.
62. 5:16And with Volvox, you can end up with thousands.
63. 5:21And more than just the sheer increase in numbers is what the daughter cells do when they are
64. 5:26newly made.
65. 5:28In the Chlamydomonas algae, they separate completely through a process called cytokinesis,
66. 5:34becoming their own individual self.
67. 5:36But with the colonial algae and on up, that cytokinesis is incomplete, leaving the cells
68. 5:43connected.
69. 5:45Those connections are further supported by an extracellular matrix made up of glycoproteins.
70. 5:52And as the number of cells increased through evolution, so too did the volume of that matrix.
71. 5:58At certain points in a Volvox’s life cycle, that transparent scaffolding can make up 99%
72. 6:05of its volume.
73. 6:07But multicellular is more than just getting big.
74. 6:11The big step to taking the volvocine algae from colony to multicellular life involves
75. 6:18putting all those cells to work--specialized work.
76. 6:23You can see the early days of specialization in the colonial algae through the development
77. 6:28of organismal polarity, which is a fancy way of saying that we can define a front and back
78. 6:34of the colony.
79. 6:36The cells in front are generally large with big eyespots.
80. 6:41And as you go to the back, the cells and eyespots get smaller.
81. 6:45These differences create a gradient not just in characteristics, but in function.
82. 6:50The larger eyespots in front mean those cells will be more sensitive to light, making them
83. 6:56responsible for directing the colony.
84. 6:59With the Volvox, this specialization transforms into a full division labor that you can actually
85. 7:05see reflected in the cells themselves.
86. 7:08Its globe-like shape features thousands of tiny cells dotting the exterior, all orbiting
87. 7:14a few large cells held in the middle.
88. 7:17Those are two distinct cell types: the thousands of cells on the outside are somatic cells,
89. 7:25decorated with flagella to help the organism move around.
90. 7:28And there may be thousands of these somatic cells, but they can’t do the most important
91. 7:34thing an organism needs to do, which is reproduce.
92. 7:38Instead, that role is given to those large cells called gonidia.
93. 7:43At some point in the Volvox’s life, those gonidia will each divide to produce a new
94. 7:49Volvox, which is essential to keeping the Volvox lineage going.
95. 7:53The one thing the gonidia can’t do is swim.
96. 7:57They don’t have the flagella for it.
97. 8:00And that’s why they need somatic cells.
98. 8:03It is a full division of labor, one that you’ll find in many forms of multicellular organisms
99. 8:09all over the planet.
100. 8:11But why?
101. 8:12Why go through all of this?
102. 8:14Well, there’s a lot of possible answers.
103. 8:16But one of the simplest answers is probably captured by this tardigrade idly grazing on
104. 8:22algae, looking a bit more like a “water cow” than a “water bear” here.
105. 8:28This Chlamydomonas algae does not stand a chance.
106. 8:32But you know who does stand a chance?
107. 8:34A pandorina.
108. 8:35It’s simply too large to get into the tardigrade’s mouth.
109. 8:40And in this moment, hundreds of millions of years of evolution--of cells dividing, of
110. 8:46being held together through incomplete cytokinesis, of biologically defining front and back--all
111. 8:52of that comes together to save this little guy’s life.
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