Saturday Science: Earth, A Biography: Another Building Block

This is actually a footnote to an earlier post on this topic. It seems that researchers may have discovered the forerunners of the means by which living cells turn raw materials into energy:

Our cells turn oxygen organic compounds, especially sugars, into energy and CO2 through a process known as cellular respiration. You may have been told in school that mitochondria, the specialized organelles which handle cellular respiration, ‘burn' food to release energy, and that's a pretty functional illustration of the process. The whole picture is, however, much larger that.

Energy-producing mechanisms in our body are often referred to as cycles - one of the most common ones is the citric acid / Krebs cycle. These are usually very complicated processes, and as such, are very difficult to wrap your head around and study them in the lab. Which is a pain if you're the kind of scientist who's trying to understand how these cycles came to be, where they first started from, and how they evolved. However, new research could offer these researchers the lucky break they need.

The processes they've discovered are, by living organism standards, pretty rudimentary:

However, the team, which also included members from the Scripps Research Institute in California and two undergrads at Furman, found two compounds which could maintain a Krebs-like cycle in experiments mimicking conditions on early Earth. Christened the HKG- and malonate-cycles, the team says these processes are likely very similar to the pre-life versions of the reactions that keep us alive today.

They're hugely less efficient than the Krebs cycle, but that was to be expected, given the lack of supportive enzymes. More importantly, however, they're based on a similar chemical blueprint - both cycles turn a molecule called glyoxylate into CO2 and other molecules in the presence of an electron-capturing agent. They're so similar, the authors' hypothesis is that biology picked them up as it was and simply tweaked and improved upon them, leading to the reactionary cycles we see today.

Given that evolution spends a lot of time adapting existing structures and processes, these two processes could very well be the beginning of metabolism --- add a few enzymes here and there as you go along, and eventually, you wind up with the Krebs cycle.

As it stands, these early processes could very well have made life possible.

I Guess I Haven't Been Paying Attention

So this morning, as I was opening the blinds on the window where all the plants are clustered, I noticed that my Encyclia tampensis is blooming.

Now, you have to understand, an orchid blooming is not something that happens fast -- being epiphytes, they have slow metabolisms.* It just goes to show how distracted I've been that this one managed to put out a stalk with four buds without my noticing.

I'm also surprised that it's blooming at all: I've had this plant for years and this is only the second time it's bloomed. The first time was the summer I was able to put it out in full sun. (They're native to Florida, ranging up into the South Carolina coast,and tend to favor sunny locations on branches and tree trunks.) I haven't been able to provide those conditions in my new place (no yard), and I'd sort of resigned myself to just letting it grow until someday. . . . (And from one small plant that I purchased years ago, it is now three rather full plants.)

The picture is close to mine, but the lip is completely purple on mine; the color of the petals and sepals is not quite as strong, but would be stronger if it had more light.

* That is, most orchids are epiphytes, at least the tropical and subtropical species. Most of our native North American orchids -- lady's slippers, ladies' tresses and the like -- are terrestrial and can be found, or once could be found, in prairies and grasslands, or in the case of lady's slippers, oak woods. (And, fun fact, there are species of Cyprepedium -- lady's slippers -- found as far north as Alaska.)

Saturday Science: Earth: A Biography: Eukaryotes, or How I Developed a Nucleus and Changed the World

We still in the Precambian era, but we've progressed to about three billion years ago, the earth's tectonic plates have formed and have started moving around, cyanobacteria have started releasing oxygen, which is going to take a while to have an effect on the types of life found on earth (like about a billion years), but it will, and we have archea, bacteria, and eukaryotes.

There's a fair amount of controversy about the relationships between these three groups. The article on the Archea at Wikipedia is fairly detailed and quite informative (and heavily referenced, so I'm taking it as fairly accurate -- it's not like it's the entry on Donald Trump or something), and has this to say about the relationships among the archea, bacteria, and eukaryotes:

The evolutionary relationship between archaea and eukaryotes remains unclear. Aside from the similarities in cell structure and function that are discussed below, many genetic trees group the two.

Complicating factors include claims that the relationship between eukaryotes and the archaeal phylum Crenarchaeota is closer than the relationship between the Euryarchaeota and the phylum Crenarchaeota[69] and the presence of archaea-like genes in certain bacteria, such as Thermotoga maritima, from horizontal gene transfer. The standard hypothesis states that the ancestor of the eukaryotes diverged early from the Archaea, and that eukaryotes arose through fusion of an archaean and eubacterium, which became the nucleus and cytoplasm; this explains various genetic similarities but runs into difficulties explaining cell structure.[73] An alternative hypothesis, the eocyte hypothesis, posits that Eukaryota emerged relatively late from the Archaea.

A recently discovered lineage of archaea, Lokiarchaeum, named for a hydrothermal vent called Loki's Castle in the Arctic Ocean, has been found to be most closely related to eukaryotes. It has been called a transitional organism between prokaryotes and eukaryotes.

(Citations removed.)

Phylogenetic tree showing the relationship between the Archaea and other domains of life. Eukaryotes are colored red, archaea green and bacteria blue. Adapted from Ciccarelli et al. (2006)

Now, this all comes with a big "but" -- whatever the eukaryotes developed from, we still don't know how it happened. Given that we're dealing with single-celled organisms, fossil remains are unlikely. Remember, the evidence that we do have for early life forms isn't comprised of actual fossils of the organisms themselves, but of "tracers" that they left behind -- graphite in zircons and fossilized stromatolites. So we can't point to a fossil of something and say "See? That's the missing link between the archea and the eukarya." So we have to infer a lot.

This article is a little dense in places, but gives a good idea of some of the important differences between prokaryotes (archea and bacteria) and eukaryotes:

There is a sharp divide in the organizational complexity of the cell between eukaryotes, which have complex intracellular compartmentalization, and even the most sophisticated prokaryotes (archaea and bacteria), which do not. A typical eukaryotic cell is about 1,000-fold bigger by volume than a typical bacterium or archaeon, and functions under different physical principles: free diffusion has little role in eukaryotic cells, but is crucial in prokaryotes. The compartmentalization of eukaryotic cells is supported by an elaborate endomembrane system and by the actin-tubulin-based cytoskeleton. There are no direct counterparts of these organelles in archaea or bacteria. The other hallmark of the eukaryotic cell is the presence of mitochondria, which have a central role in energy transformation and perform many additional roles in eukaryotic cells, such as in signaling and cell death.

The conservation of the major features of cellular organization and the existence of a large set of genes that are conserved across eukaryotes leave no doubt that all extant eukaryotic forms evolved from a last eukaryote common ancestor (LECA; see below). All eukaryotes that have been studied in sufficient detail possess either mitochondria or organelles derived from mitochondria, so it is thought that LECA already possessed mitochondria (see below). Plants and many unicellular eukaryotes also have another type of organelle, plastids.

(Citations removed.)

So you can see that we're dealing with a quantum leap in complexity and functionality. And note also that mitochodria, which play such an essential role in cell metabolism, also, according to some theories, represent what we can only describe as a symbiotic relationship:

The endosymbiotic hypothesis for the origin of mitochondria (and chloroplasts) suggests that mitochondria are descended from specialized bacteria (probably purple nonsulfur bacteria) that somehow survived endocytosis by another species of prokaryote or some other cell type, and became incorporated into the cytoplasm. The ability of symbiont bacteria to conduct cellular respiration in host cells that relied on glycosis and fermentation would have provided a considerable evolutionary advantage. Similarly, host cells with symbiont bacteria capable of photosynthesis would also have an advantage. In both cases, the number of environments in which the cells could survive would have been greatly expanded.

Here's a nice comparison of your basic, generic prokaryotic cell and your basic, generic eukaryotic cell:

You can see that eukaryotes, even the single-celled variety, are much more complex than their forebears. They also, in evolutionary terms, have a huge advantage, being able to adapt to a greater range of environments. Granted, there aren't nearly as many environments on Earth as there will be later, but eukaryotes were able to adapt to more of them -- a trait that will continue.

We're going to take a leap in time for next time because for the next couple billion years, not much was happening. Except sex. Brace yourself.