Evolution of multicellular eggs

Evolution of multicellular eggs

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Which animals where the first in which ova were not simply released, but instead provided with some additional nutrition and/or protection in the form of a larger egg?

Cells do not always part after mitosis, but sometimes stay together to form multicellular organisms. This increases their size, and hence provides a defense against predators. Unfortunately, it is not possible simply to increase the size of cell because the really big cell will have less surface (in relation to the volume), therefore it will have multiple difficulties with photosynthesis, respiration and other processes which relate with surface of cell. But many cells together will make surface big enough (Figure (PageIndex<1>)). Multicellular organism has two modes of growth: scaling the body and multiplying cells.

Figure (PageIndex<1>) Origin of multicellularity. It is not feasible just to enlarge cell, surface is too small. But if cells do not part after mitosis, they might form the body which is big enough to escape from predators. This also provide with new mode of growth and possibility of the division of labor (colored cells).

Multicellularity allows these cells also to divide the labor and cooperate. This is extremely important for the future evolution.

Cells in the multicellular body are not connected forever. Sometimes, one or few cells escape and start a new body. This body will be exact copy (clone) of the previous one (vegetative reproduction). It is also possible that when these &ldquoescaped cells&rdquo go the different route: they become &ldquosex delegates&rdquo, gametes. All gametes want syngamy, and these cells will search for the partner of the same species but with another genotype. In case of heterogamy and oogamy, it is easy to recognize because genders will provide a hint: male will search for the female. In case of isogamy, gametes search for the partner with different surface proteins. After they finally mate, a diploid cell (zygote) appears. Zygote may winter and then divide meiotically. This is the simplest life cycle of multicellular organism (Figure (PageIndex<2>)), quite similar to the cycle discussed above for unicellular organism.

Figure (PageIndex<2>) Most ancient life cycle of the multicellular organism. Zygote does not grow, it divides meiotically. Somatic (&ldquogrey&rdquo) cells are going to die, only germ cells transfer their DNA to future generations.

However, frequently zygote starts to grow and divide mitotically, making the diploid body. There are two reasons to make multicellular body out of zygote without meiosis: (a) because in can and (b) because diploid is better. &ldquoIt can&rdquo because zygote already contains DNA program about how to build multicellular body. Why diploid is better, explained in next section.

If multicellular organism consists of diploid cells ((2n)), we will use the neutral term diplont. Multicellular organisms with haploids cells ((n)) are haplonts.

&ldquoEscaped cells&rdquo, &ldquosex delegates&rdquo, or mother cells of gametes from the above is a first stage of the division of labor when cells are separating into two types, germ cells and somatic cells. Somatic cells are those which will eventually die, but germ cells are capable of giving offspring. Having germ cells is not absolutely necessary for multicellular organisms, but most of them have well separated germ lines. Thus, origin of death is directly connected with this separation: somatic cells are not needed for future generations. Unicellular organisms are potentially immortal, and same are cancer cells which also escape from organism (but they cannot make the new one).

Life cycle of multicellular organism could be described starting from haplont (Figure (PageIndex<3>)). When environment conditions are favorable, it has vegetative reproduction. One variant of vegetative reproduction is that cell (mitospore) separates itself from a haplont, then divides into more cells and becomes a new haplont. Sometimes, whole chunks are separated and grow into new haplonts. When conditions change, haplont may start the sexual reproduction: syngamy. In syngamy, one gamete separates from the haplont and unites with a gamete from another haplont. Together, gametes form a zygote. This zygote might go straight to meiosis (as it happens in unicellular eukaryotes) but more frequently, zygote will grow, divide mitotically and finally becomes a diplont. This diplont might be superficially almost identical to haplont but every cell of it contains diploid nucleus (every chromosome has a pair). Diplont (similarly to haplont) may reproduce itself vegetatively (make clones): cell separates itself from a diplont, then divides mitotically into more cells and becomes a new diplont.

The diplont is also capable for asexual reproduction: there could be a cell separates itself from a diplont and divides with meiosis creating four spores, each of them will grow into haplont.

Figure (PageIndex<3>) General life cycle. Haploid part is on the left, diploid on the right, syngamy on the top, meiosis on the bottom. "M" letter is used to label mitosis.

Sporic, Zygotic and Gametic Life Cycles

The life cycle described above is the sporic life cycle (Figure (PageIndex<4>)). Organisms with sporic life cycle have both diplont and haplont, equally or unequally developed.

Figure (PageIndex<4>) Sporic life cycle. Overview. Haploid part is on the left, diploid on the right, syngamy on the top, meiosis on the bottom.

In all, there are three types of life cycles: sporic, zygotic, which is the most similar to unicellular and most primitive and gametic, which is used by animals and a few protists (Figure (PageIndex<5>)). The zygotic life cycle starts with syngamy and goes to meiosis. It has no diplont. Gametic life cycle goes from meiosis to syngamy. It has no haplont.

Protists have all three types of life cycles whereas higher groups have only one. Animals exhibit gametic cycle, whereas plants(_2) retained the more primitive sporic cycle.

Evolution of Life Cycles

The most striking difference between unicellular and multicellular life cycles is that zygote of multicellular organism may start to make diploid body (diplont) which sometimes is visually almost identical to haplont. This is because in the evolutionary perspective, diplonts are &ldquobetter&rdquo than haplonts. Frequent situation of gene dominance allows only one variant (allele) of the gene to work, that may save organism from lethal mutations. An increased number of genes could help to make more proteins. A third reason is that diplonts&rsquo genomes are more diverse. One gene may be able to withstand one group of conditions, and the other variant may have a different set of possible conditions. Therefore, diplont is able to take advantage of the capabilities of both genetic variants.

Figure (PageIndex<5>) The evolution of life cycles (green arrows represent five evolutionary transitions) from unicellular zygotic to multicellular gametic through different variants of sporic cycles.

As a consequence, the evolution of life cycles goes from zygotic (similar to unicellular) to the sporic cycle (Figure (PageIndex<6>)), and then to the more and more expressed domination of diplont, and finally to the complete reduction of haplont, gametic life cycle. It is still an open question how zygotic protists evolved to the sporic side. Most probably, zygote (which is diploid by definition) did not want to divide meiotically. Instead, it grows (which is seen in some protists) and divides mitotically, giving birth to the diplont. This is how first sporic cycle started. The last step of this evolutionary chain was a complete reduction of haplont: after meiosis, spores were replaced with gametes which immediately go to syngamy.

Life Cycle of Vegetabilia

Ancestors of Vegetabilia (plants(_2)) were green algae with zygotic life cycle. It could be imagined that their zygote started to grow because these organisms inhabited shallow waters and want their spores to be distributed with a wind. One way for this to happen is to have the spores on the stalk of the plant. This is probably the reason

Figure (PageIndex<6>)The evolution of life cycles (green arrows represent five evolutionary transi- tions) from unicellular zygotic to multicellular gametic through different variants of sporic cycles.

of zygote growth: primordial diplonts of plants(_2) were simply sporangia, structures bearing spores. Then the benefits of diploid condition described above started to appear, and these primitive plants went onto the road of haplont reduction. However, some Vegetabilia (liverworts, mosses and hornworts), still have haplont domination. This is probably because their haplonts are poikilohydric (it is explained in next chapters), adaptation which is beneficial for small plants.

Life cycle of plants(_2) is sporic, but the science tradition uses plant-related names for the stages. The cycle (Figure (PageIndex<7>)) begins with a diplont called a sporophyte, which produces spores. Sporophyte bears a sporangium, inside which mother cell of spores uses meiosis to make spores. The spores germinate and grow into haplont called gametophyte. Gametophyte produces gametes, specifically a spermatozoa (or simply &ldquosperms&rdquo) and an oocyte (egg cell). These gametes are developed in special organs&mdashgametangia. Gametangium which contains male gametes (sperms) is called antheridium, and female gametangium is archegonium, the last normally contains only one egg cell (oocyte).

By syngamy (oogamy in this case), the two gametes form a zygote. Next, a young sporophyte grows on the gametophyte, and finally, the cycle starts again. Again, sporophyte of Vegetabilia starts its life as a parasite on gametophyte. Even flowering plants have this stage called embryo. Maybe, this is why the gametophyte of plants(_2) has never been reduced completely to transform their cycle into gametic. Even in most advanced plant lineages, their male (which makes only sperms) and female gametophytes have minimum 3 and 4 cells, respectively, but not 0!

Figure (PageIndex<7>) Life cycle of land plants. Red color is used for innovations, comparing with previous (general) life cycle scheme.

How did multicellular life evolve?

Cells of Dictyostelium purpureum, a common soil microbe, streaming to form a multicellular fruiting body. Credit: Natasha Mehdiabadi/Rice University

Scientists are discovering ways in which single cells might have evolved traits that entrenched them into group behavior, paving the way for multicellular life. These discoveries could shed light on how complex extraterrestrial life might evolve on alien worlds.

Researchers detailed these findings in the Oct. 24 issue of the journal Science.

The first known single-celled organisms appeared on Earth about 3.5 billion years ago, roughly a billion years after Earth formed. More complex forms of life took longer to evolve, with the first multicellular animals not appearing until about 600 million years ago.

The evolution of multicellular life from simpler, unicellular microbes was a pivotal moment in the history of biology on Earth and has drastically reshaped the planet's ecology. However, one mystery about multicellular organisms is why cells did not return back to single-celled life.

"Unicellularity is clearly successful—unicellular organisms are much more abundant than multicellular organisms, and have been around for at least an additional 2 billion years," said lead study author Eric Libby, a mathematical biologist at the Santa Fe Institute in New Mexico. "So what is the advantage to being multicellular and staying that way?"

The answer to this question is usually cooperation, as cells benefitted more from working together than they would from living alone. However, in scenarios of cooperation, there are constantly tempting opportunities "for cells to shirk their duties—that is, cheat," Libby said.

"As an example, consider an ant colony where only the queen is laying eggs and the workers, who cannot reproduce, must sacrifice themselves for the colony," Libby said. "What prevents the ant worker from leaving the colony and forming a new colony? Well, obviously the ant worker cannot reproduce, so it cannot start its own colony. But if it got a mutation that enabled it to do that, then this would be a real problem for the colony. This kind of struggle is prevalent in the evolution of multicellularity because the first multicellular organisms were only a mutation away from being strictly unicellular."

When social amoeba Dictyostelium discoideum starves, it forms a multicellular body. Credit: Scott Solomon

Experiments have shown that a group of microbes that secretes useful molecules that all members of the group can benefit from can grow faster than groups that do not. But within that group, freeloaders that do not expend resources or energy to secrete these molecules grow fastest of all. Another example of cells that grow in a way that harms other members of their groups are cancer cells, which are a potential problem for all multicellular organisms.

Indeed, many primitive multicellular organisms probably experienced both unicellular and multicellular states, providing opportunities to forego a group lifestyle. For example, the bacterium Pseudomonas fluorescens rapidly evolves to generate multicellular mats on surfaces to gain better access to oxygen. However, once a mat has formed, unicellular cheats have an incentive to not produce the glue responsible for mat formation, ultimately leading to the mat's destruction.

To solve the mystery of how multicellular life persisted, scientists are suggesting what they call "ratcheting mechanisms." Ratchets are devices that permit motion in just one direction. By analogy, ratcheting mechanisms are traits that provide benefits in a group context but are detrimental to loners, ultimately preventing a reversion to a single-celled state, said Libby and study co-author William Ratcliff at the Georgia Institute of Technology in Atlanta.

In general, the more a trait makes cells in a group mutually reliant, the more it serves as a ratchet. For instance, groups of cells may divide labor so that some cells grow one vital molecule while other cells grow a different essential compound, so these cells do better together than apart, an idea supported by recent experiments with bacteria.

Ratcheting can also explain the symbiosis between ancient microbes that led to symbionts living inside cells, such as the mitochondria and chloroplasts that respectively help their hosts make use of oxygen and sunlight. The single-celled organisms known as Paramecia do poorly when experimentally derived of photosynthetic symbionts, and in turn symbionts typically lose genes that are required for life outside their hosts.

These ratcheting mechanisms can lead to seemingly nonsensical results. For instance, apoptosis, or programmed cell death, is a process by which a cell essentially undergoes suicide. However, experiments show that higher rates of apoptosis can actually have benefits. In large clusters of yeast cells, apoptotic cells act like weak links whose death allows small clumps of yeast cells to break free and go on to spread elsewhere where they might have more room and nutrients to grow.

Groups of yeast cells. If key cells die a programmed death, these groups can separate. Credit: E. Libby et al., PLOS Computational Biology

"This advantage does not work for single cells, which meant that any cell that abandoned the group would suffer a disadvantage," Libby said. "This work shows that a cell living in a group can experience a fundamentally different environment than a cell living on its own. The environment can be so different that traits disastrous for a solitary organism, like increased rates of death, can become advantageous for cells in a group."

When it comes to what these findings mean in the search for alien life, Libby said this research suggests that extraterrestrial behavior might appear odd until one better understands that an organism may be a member of a group.

"Organisms in communities can adopt behaviors that would appear bizarre or counterintuitive without proper consideration of their communal context," Libby said. "It is essentially a reminder that a puzzle piece is a puzzle until you know how it fits into a larger context."

A fossil of a 600 million-year-old multicellular organism displays unexpected evidence of complexity. Credit: Virginia Tech

Libby and his colleagues plan to identify other ratcheting mechanisms.

"We also have some experiments in the works to calculate the stability provided by some possible ratcheting traits," Libby said.

Did Earth's early rise in oxygen support the evolution of multicellular life—or suppress it?

Credit: Pixabay/CC0 Public Domain

Scientists have long thought that there was a direct connection between the rise in atmospheric oxygen, which started with the Great Oxygenation Event 2.5 billion years ago, and the rise of large, complex multicellular organisms.

That theory, the "Oxygen Control Hypothesis," suggests that the size of these early multicellular organisms was limited by the depth to which oxygen could diffuse into their bodies. The hypothesis makes a simple prediction that has been highly influential within both evolutionary biology and geosciences: Greater atmospheric oxygen should always increase the size to which multicellular organisms can grow.

It's a hypothesis that's proven difficult to test in a lab. Yet a team of Georgia Tech researchers found a way—using directed evolution, synthetic biology, and mathematical modeling—all brought to bear on a simple multicellular lifeform called a 'snowflake yeast." The results? Significant new information on the correlations between oxygenation of the early Earth and the rise of large multicellular organisms—and it's all about exactly how much O2 was available to some of our earliest multicellular ancestors.

"The positive effect of oxygen on the evolution of multicellularity is entirely dose-dependent—our planet's first oxygenation would have strongly constrained, not promoted, the evolution of multicellular life," explains G. Ozan Bozdag, research scientist in the School of Biological Sciences and the study's lead author. "The positive effect of oxygen on multicellular size may only be realized when it reaches high levels."

"Oxygen suppression of macroscopic multicellularity" is published in the May 14, 2021 edition of the journal Nature Communications. Bozdag's co-authors on the paper include Georgia Tech researchers Will Ratcliff, associate professor in the School of Biological Sciences Chris Reinhard, associate professor in the School of Earth and Atmospheric Sciences Rozenn Pineau, Ph.D. student in the School of Biological Sciences and the Interdisciplinary Graduate Program in Quantitative Biosciences (QBioS) along with Eric Libby, assistant professor at Umea University in Sweden and the Santa Fe Institute in New Mexico.

Directing yeast to evolve in record time

"We show that the effect of oxygen is more complex than previously imagined. The early rise in global oxygen should in fact strongly constrain the evolution of macroscopic multicellularity, rather than selecting for larger and more complex organisms," notes Ratcliff.

"People have long believed that the oxygenation of Earth's surface was helpful—some going so far as to say it is a precondition—for the evolution of large, complex multicellular organisms," he adds. "But nobody has ever tested this directly, because we haven't had a model system that is both able to undergo lots of generations of evolution quickly, and able to grow over the full range of oxygen conditions," from anaerobic conditions up to modern levels.

The researchers were able to do that, however, with snowflake yeast, simple multicellular organisms capable of rapid evolutionary change. By varying their growth environment, they evolved snowflake yeast for over 800 generations in the lab with selection for larger size.

The results surprised Bozdag. "I was astonished to see that multicellular yeast doubled their size very rapidly when they could not use oxygen, while populations that evolved in the moderately oxygenated environment showed no size increase at all," he says. "This effect is robust—even over much longer timescales."

Size—and oxygen levels—matter for multicellular growth

In the team's research, "large size easily evolved either when our yeast had no oxygen or plenty of it, but not when oxygen was present at low levels," Ratcliff says. "We did a lot more work to show that this is actually a totally predictable and understandable outcome of the fact that oxygen, when limiting, acts as a resource—if cells can access it, they get a big metabolic benefit. When oxygen is scarce, it can't diffuse very far into organisms, so there is an evolutionary incentive for multicellular organisms to be small—allowing most of their cells access to oxygen—a constraint that is not there when oxygen simply isn't present, or when there's enough of it around to diffuse more deeply into tissues."

Ratcliff says not only does his group's work challenge the Oxygen Control Hypothesis, it also helps science understand why so little apparent evolutionary innovation was happening in the world of multicellular organisms in the billion years after the Great Oxygenation Event. Ratcliff explains that geologists call this period the "Boring Billion" in Earth's history—also known as the Dullest Time in Earth's History, and Earth's Middle Ages—a period when oxygen was present in the atmosphere, but at low levels, and multicellular organisms stayed relatively small and simple.

Bozdag adds another insight into the unique nature of the study. "Previous work examined the interplay between oxygen and multicellular size mainly through the physical principles of gas diffusion," he says. "While that reasoning is essential, we also need an inclusive consideration of principles of Darwinian evolution when studying the origin of complex multicellular life on our planet." Finally being able to advance organisms through many generations of evolution helped the researchers accomplish just that, Bozdag adds.

The Evolution of Metabolism

Because cells originated in a sea of organic molecules, they were able to obtain food and energy directly from their environment. But such a situation is self-limiting, so cells needed to evolve their own mechanisms for generating energy and synthesizing the molecules necessary for their replication. The generation and controlled utilization of metabolic energy is central to all cell activities, and the principal pathways of energy metabolism (discussed in detail in Chapter 2) are highly conserved in present-day cells. All cells use adenosine 5-triphosphate (ATP) as their source of metabolic energy to drive the synthesis of cell constituents and carry out other energy-requiring activities, such as movement (e.g., muscle contraction). The mechanisms used by cells for the generation of ATP are thought to have evolved in three stages, corresponding to the evolution of glycolysis, photosynthesis, and oxidative metabolism (Figure 1.5). The development of these metabolic pathways changed Earth's atmosphere, thereby altering the course of further evolution.

Figure 1.5

Generation of metabolic energy. Glycolysis is the anaerobic breakdown of glucose to lactic acid. Photosynthesis utilizes energy from sunlight to drive the synthesis of glucose from CO2 and H2O, with the release of O2 as a by-product. The O2 released by (more. )

In the initially anaerobic atmosphere of Earth, the first energy-generating reactions presumably involved the breakdown of organic molecules in the absence of oxygen. These reactions are likely to have been a form of present-day glycolysis—the anaerobic breakdown of glucose to lactic acid, with the net energy gain of two molecules of ATP. In addition to using ATP as their source of intracellular chemical energy, all present-day cells carry out glycolysis, consistent with the notion that these reactions arose very early in evolution.

Glycolysis provided a mechanism by which the energy in preformed organic molecules (e.g., glucose) could be converted to ATP, which could then be used as a source of energy to drive other metabolic reactions. The development of photosynthesis is generally thought to have been the next major evolutionary step, which allowed the cell to harness energy from sunlight and provided independence from the utilization of preformed organic molecules. The first photosynthetic bacteria, which evolved more than 3 billion years ago, probably utilized H2S to convert CO2 to organic molecules𠅊 pathway of photosynthesis still used by some bacteria. The use of H2O as a donor of electrons and hydrogen for the conversion of CO2 to organic compounds evolved later and had the important consequence of changing Earth's atmosphere. The use of H2O in photosynthetic reactions produces the by-product free O2 this mechanism is thought to have been responsible for making O2 abundant in Earth's atmosphere.

The release of O2 as a consequence of photosynthesis changed the environment in which cells evolved and is commonly thought to have led to the development of oxidative metabolism. Alternatively, oxidative metabolism may have evolved before photosynthesis, with the increase in atmospheric O2 then providing a strong selective advantage for organisms capable of using O2 in energy-producing reactions. In either case, O2 is a highly reactive molecule, and oxidative metabolism, utilizing this reactivity, has provided a mechanism for generating energy from organic molecules that is much more efficient than anaerobic glycolysis. For example, the complete oxidative breakdown of glucose to CO2 and H2O yields energy equivalent to that of 36 to 38 molecules of ATP, in contrast to the 2 ATP molecules formed by anaerobic glycolysis. With few exceptions, present-day cells use oxidative reactions as their principal source of energy.


Alcock, John. Animal Behavior: An Evolutionary Approach, 6th ed. Sunderland, MA: Sinauer Associates, Inc., 1998.

Catton, Chris, and James Gray. Sex in Nature. New York: Facts on File Publications, 1985.

Eberhard, William. Female Control: Sexual Selection by Cryptic Female Choice. Princeton, NJ: Princeton University Press, 1996.

Forsyth, Adrian. A Natural History of Sex. Shelburne, VT: Chapters Publishing, 1986.

Gilbert, Scott. Developmental Biology, 5th ed. Sunderland, MA: Sinauer Associates, Inc., 1997.

Eukaryotic Life Cycles

Regardless of an organism’s ecology, there are three fundamental steps to sexual reproduction:

  • Gametogenesis: making gametes
  • Mating: getting gametes together
  • Fertilization: fusing gametes into one cell

Life cycles: Different organisms accomplish these three steps in different ways and at different times in their life cycles (note that a change in ploidy is always required). All life cycles involve a haploid (1 complete set of chromosomes) and diploid (2 complete sets of chromosomes) stage, but they vary in how and when in the life cycle these stages occur.

In the diplontic life cycle, which is typical of animals, the mature, multicellular organism is diploid (2n). In the haplontic life cycle, which is typical of some algae, and fungi, the mature, multicellular organism is haploid. In the alternation of generations life cycle, which is typical of plants, there are two mature, multicellular organisms: one haploid, and one diploid. These life cycles are illustrated below, with details about each life cycle provided in the captions. Before you read through the details of these diagrams, let’s define some terms:

  • Gamete: a mature haploid male or female germ cell that is able to unite with another of the opposite sex in sexual reproduction to form a zygote
  • Spore: a small, typically one-celled, reproductive unit capable of giving rise to a new individual without sexual fusion

Gametes are always haploid, and spores are usually haploid (spores are haploid in all contexts we will consider in this class and in the diagrams below).

Eukaryotic life cycles. A: Haplontic life cycle. The mature, multicellular organism is haploid and produces haploid gametes via mitosis, which fuse into a diploid zygote. The zygote immediately undergoes meiosis to produce haploid spores, which develop into mature multicellular haploid individuals. B: Diplontic life cycle. The mature, multicellular organism is diploid and produces haploid gametes via meiosis, which fuse into a diploid zygote. The zygote undergoes development into a mature multicellular diploid organism. C: Alternation of Generations. There is a mature multicellular haploid stage and a mature mulitcellular diploid stage. The multicelluar haploid stage (the gametophyte) produces gametes via mitosis which fuse to form a diploid zygote. The zygote develops into a mature multicellular diploid individual (the sporophyte), which produces haploid spores via meiosis. The haploid spores then develop into a mature multicellular haploid individual.. Image credit: CK-12 Foundation,

Alternation of generations is often the most confusing life cycle to understand, because it is so different from the diplontic life cycle (what we think of as “normal”.) The video below illustrates the differences between the diplontic and alternation of generations life cycles:

Billion-year-old Fossil Reveals Missing link In The Evolution Of Animals

A billion year old fossil, which provides a new link in the evolution of animals, has been discovered in the Scottish Highlands.

A team of scientists, led by the University of Sheffield in the UK and Boston College in the USA, has found a microfossil which contains two distinct cell types and could be the earliest multicellular animal ever recorded.

The fossil reveals new insight into the transition of single celled organisms to complex multicellular animals. Modern single celled holozoa include the most basal living animals, the fossil discovered shows an organism which lies somewhere between single cell and multicellular animals.

The fossil has been described and formally named Bicellum Brasieri in a new research paper published in Current Biology.

Professor Charles Wellman, one of the lead investigators of the research, from the University of Sheffield's Department of Animal and Plant Sciences, said: "The origins of complex multicellularity and the origin of animals are considered two of the most important events in the history of life on Earth, our discovery sheds new light on both of these.

"We have found a primitive spherical organism made up of an arrangement of two distinct cell types, the first step towards a complex multicellular structure, something which has never been described before in the fossil record.

"The discovery of this new fossil suggests to us that the evolution of multicellular animals had occurred at least one billion years ago and that early events prior to the evolution of animals may have occurred in freshwater like lakes rather than the ocean."

Professor Paul Strother, lead investigator of the research from Boston College, said: "Biologists have speculated that the origin of animals included the incorporation and repurposing of prior genes that had evolved earlier in unicellular organisms.

"What we see in Bicellum is an example of such a genetic system, involving cell-cell adhesion and cell differentiation that may have been incorporated into the animal genome half a billion years later."

The fossil was found at Loch Torridon in the Northwest Scottish Highlands. Scientists were able to study the fossil due to its exceptional preservation, allowing them to analyse it at a cellular and subcellular level.

The team hope to now examine the Torridonian deposits for more interesting fossils which could provide more insight into the evolution of multicellular organisms.

The research is mainly funded by the UK Natural Environment Research Council (NERC).

15 Introduction to the Cellular Basis of Inheritance

Figure 1: Each of us, like these other large multicellular organisms, begins life as a fertilized egg. After trillions of cell divisions, each of us develops into a complex, multicellular organism. (credit a: modification of work by Frank Wouters credit b: modification of work by Ken Cole, USGS credit c: modification of work by Martin Pettitt)

The ability to reproduce in kind is a basic characteristic of all living things. In kind means that the offspring of any organism closely resembles its parent or parents. Hippopotamuses give birth to hippopotamus calves Monterey pine trees produce seeds from which Monterey pine seedlings emerge and adult flamingos lay eggs that hatch into flamingo chicks. In kind does not generally mean exactly the same. While many single-celled organisms and a few multicellular organisms can produce genetically identical clones of themselves through mitotic cell division, many single-celled organisms and most multicellular organisms reproduce regularly using another method.

Sexual reproduction is the production by parents of haploid cells and the fusion of a haploid cell from each parent to form a single, unique diploid cell. In multicellular organisms, the new diploid cell will then undergo mitotic cell divisions to develop into an adult organism. A type of cell division called meiosis leads to the haploid cells that are part of the sexual reproductive cycle. Sexual reproduction, specifically meiosis and fertilization, introduces variation into offspring that may account for the evolutionary success of sexual reproduction. The vast majority of eukaryotic organisms can or must employ some form of meiosis and fertilization to reproduce.