The idea that animals arose from a colony of choanoflagellate-like cells implies that cell differentiation evolved after multicellularity did. The first complication came in , when a group of scientists, in an effort to more precisely map out the evolutionary relationships among animals on the tree of life, identified comb jellies rather than sponges as the earliest animals.
The finding generated controversy. Video: This time-lapse video shows cell behavior in a juvenile Amphimedon queenslandica sponge. Subsequent discoveries continued to fuel the debate over which animal group came first.
And some studies uncovered overlooked differences between choanoflagellates and sponge choanocytes. Scientists also began to realize that choanoflagellates and two closely related unicellular groups all have complex life cycles that proceed through various cell states. These states essentially act as different cell types — but rather than all existing side by side as in a multicellular organism, they arise sequentially in a single cell.
And during those life cycles, all three of these protists spend part of their lives in a form that borders on something like primitive multicellularity. Choanoflagellates have a colonial form; the second protist group has amoeba-like cells that aggregate; the cells of the third group grow to have hundreds of nuclei. Back in , the Russian biologist Alexey Zakhvatkin had proposed that multicellular animals evolved when temporally differentiating cells formed colonies and began to commit to particular stages in their life cycles, allowing a few cell types to exist at once.
Ruiz-Trillo and his colleagues provided further evidence for this so-called temporal-to-spatial transition. In a series of studies, they showed that certain families of regulatory proteins supposedly unique to animals, including those involved in cell differentiation, were actually already present in their far more ancient unicellular relatives.
To do so, they examined the gene expression in choanocytes and other kinds of sponge cells, then compared those findings with published data on choanoflagellates and two other protists. They expected to establish that sponge choanocytes had gene expression profiles most like those of choanoflagellates. Instead, they found that another type of sponge cell did.
That cell type, called an archaeocyte, acts like a stem cell for the sponge: It can differentiate into any other cell type the animal might need. Some of the gene expression patterns in archaeocytes are significantly similar to those of the protists during particular life cycle stages, according to Bernard Degnan. Moreover, the choanocytes seemed to be unexpectedly transient.
In the shared ancestry of choanoflagellates and sponges there could have been something like an archaeocyte or a pluripotent stem cell. According to some experts, we can think of the single-celled organisms that came before animals as stem cells of sorts: They could go on dividing forever, and they could perform a variety of functions, including reproduction. Other early animals, such as jellyfish, show a great deal of that seemingly ancestral plasticity as well.
Note: Content may be edited for style and length. Science News. Genomic data do not support comb jellies as the sister group to all other animals. ScienceDaily, 1 December Animal evolution: Sponges really are oldest animal phylum. Retrieved November 14, from www. Sponges have the genes involved in neuronal function in higher animals. But if sponges don't have brains, what is the role of these? Their work finds strong evidence that sponges - not ScienceDaily shares links with sites in the TrendMD network and earns revenue from third-party advertisers, where indicated.
Print Email Share. Boy or Girl? For approximately 20 species of marine dinoflagellates, population explosions also called blooms during the summer months can tint the ocean with a muddy red color. This phenomenon is called a red tide, and it results from the abundant red pigments present in dinoflagellate plastids. In large quantities, these dinoflagellate species secrete an asphyxiating toxin that can kill fish, birds, and marine mammals. Red tides can be massively detrimental to commercial fisheries, and humans who consume these protists may become poisoned.
Figure 5. Bioluminescence is emitted from dinoflagellates in a breaking wave, as seen from the New Jersey coast. The apicomplexan protists are so named because their microtubules, fibrin, and vacuoles are asymmetrically distributed at one end of the cell in a structure called an apical complex Figure 6.
The apical complex is specialized for entry and infection of host cells. Indeed, all apicomplexans are parasitic. This group includes the genus Plasmodium , which causes malaria in humans.
Apicomplexan life cycles are complex, involving multiple hosts and stages of sexual and asexual reproduction. Figure 6. They have a characteristic apical complex that enables them to infect host cells.
The ciliates, which include Paramecium and Tetrahymena , are a group of protists 10 to 3, micrometers in length that are covered in rows, tufts, or spirals of tiny cilia.
By beating their cilia synchronously or in waves, ciliates can coordinate directed movements and ingest food particles. Certain ciliates have fused cilia-based structures that function like paddles, funnels, or fins. Ciliates also are surrounded by a pellicle, providing protection without compromising agility.
The genus Paramecium includes protists that have organized their cilia into a plate-like primitive mouth, called an oral groove, which is used to capture and digest bacteria Figure 7. Food captured in the oral groove enters a food vacuole, where it combines with digestive enzymes. Waste particles are expelled by an exocytic vesicle that fuses at a specific region on the cell membrane, called the anal pore. In addition to a vacuole-based digestive system, Paramecium also uses contractile vacuoles , which are osmoregulatory vesicles that fill with water as it enters the cell by osmosis and then contract to squeeze water from the cell.
Figure 7. Paramecium has a primitive mouth called an oral groove to ingest food, and an anal pore to excrete it. Contractile vacuoles allow the organism to excrete excess water. Cilia enable the organism to move. Watch the video of the contractile vacuole of Paramecium expelling water to keep the cell osmotically balanced. Paramecium has two nuclei, a macronucleus and a micronucleus, in each cell. The micronucleus is essential for sexual reproduction, whereas the macronucleus directs asexual binary fission and all other biological functions.
The process of sexual reproduction in Paramecium underscores the importance of the micronucleus to these protists. Paramecium and most other ciliates reproduce sexually by conjugation. This process begins when two different mating types of Paramecium make physical contact and join with a cytoplasmic bridge Figure 8.
The diploid micronucleus in each cell then undergoes meiosis to produce four haploid micronuclei. Three of these degenerate in each cell, leaving one micronucleus that then undergoes mitosis, generating two haploid micronuclei. The cells each exchange one of these haploid nuclei and move away from each other. A similar process occurs in bacteria that have plasmids. Fusion of the haploid micronuclei generates a completely novel diploid pre-micronucleus in each conjugative cell.
This pre-micronucleus undergoes three rounds of mitosis to produce eight copies, and the original macronucleus disintegrates. Four of the eight pre-micronuclei become full-fledged micronuclei, whereas the other four perform multiple rounds of DNA replication and go on to become new macronuclei.
Two cell divisions then yield four new Paramecia from each original conjugative cell. Figure 8. The complex process of sexual reproduction in Paramecium creates eight daughter cells from two original cells. Each cell has a macronucleus and a micronucleus. During sexual reproduction, the macronucleus dissolves and is replaced by a micronucleus. Figure 9. This stramenopile cell has a single hairy flagellum and a secondary smooth flagellum. The other subgroup of chromalveolates, the stramenopiles, includes photosynthetic marine algae and heterotrophic protists.
Many stramenopiles also have an additional flagellum that lacks hair-like projections Figure 9. Members of this subgroup range in size from single-celled diatoms to the massive and multicellular kelp. The diatoms are unicellular photosynthetic protists that encase themselves in intricately patterned, glassy cell walls composed of silicon dioxide in a matrix of organic particles Figure These protists are a component of freshwater and marine plankton. Most species of diatoms reproduce asexually, although some instances of sexual reproduction and sporulation also exist.
Some diatoms exhibit a slit in their silica shell, called a raphe. By expelling a stream of mucopolysaccharides from the raphe, the diatom can attach to surfaces or propel itself in one direction. During periods of nutrient availability, diatom populations bloom to numbers greater than can be consumed by aquatic organisms. The excess diatoms die and sink to the sea floor where they are not easily reached by saprobes that feed on dead organisms.
As a result, the carbon dioxide that the diatoms had consumed and incorporated into their cells during photosynthesis is not returned to the atmosphere. The biological carbon pump is a crucial component of the carbon cycle that maintains lower atmospheric carbon dioxide levels.
Figure Assorted diatoms, visualized here using light microscopy, live among annual sea ice in McMurdo Sound, Antarctica. Gordon T. Like diatoms, golden algae are largely unicellular, although some species can form large colonies. Their characteristic gold color results from their extensive use of carotenoids, a group of photosynthetic pigments that are generally yellow or orange in color.
Golden algae are found in both freshwater and marine environments, where they form a major part of the plankton community. The brown algae are primarily marine, multicellular organisms that are known colloquially as seaweeds. Giant kelps are a type of brown algae. Some brown algae have evolved specialized tissues that resemble terrestrial plants, with root-like holdfasts, stem-like stipes, and leaf-like blades that are capable of photosynthesis.
The stipes of giant kelps are enormous, extending in some cases for 60 meters. A variety of algal life cycles exists, but the most complex is alternation of generations, in which both haploid and diploid stages involve multicellularity.
Compare this life cycle to that of humans, for instance. Haploid gametes produced by meiosis sperm and egg combine in fertilization to generate a diploid zygote that undergoes many rounds of mitosis to produce a multicellular embryo and then a fetus.
However, the individual sperm and egg themselves never become multicellular beings. Terrestrial plants also have evolved alternation of generations. In the brown algae genus Laminaria , haploid spores develop into multicellular gametophytes, which produce haploid gametes that combine to produce diploid organisms that then become multicellular organisms with a different structure from the haploid form Figure Certain other organisms perform alternation of generations in which both the haploid and diploid forms look the same.
Several species of brown algae, such as the Laminaria shown here, have evolved life cycles in which both the haploid gametophyte and diploid sporophyte forms are multicellular. The gametophyte is different in structure than the sporophyte. A saprobic oomycete engulfs a dead insect. The oomycetes are characterized by a cellulose-based cell wall and an extensive network of filaments that allow for nutrient uptake.
As diploid spores, many oomycetes have two oppositely directed flagella one hairy and one smooth for locomotion. The oomycetes are nonphotosynthetic and include many saprobes and parasites. The saprobes appear as white fluffy growths on dead organisms Figure Most oomycetes are aquatic, but some parasitize terrestrial plants.
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