Long before sharks and salmon were even a twinkle in Mother Nature’s eye, strange-looking fishes patrolled the ancient seas. They made their living in many ways, such as filtering through mud, but none bit into their food–these were the days before fish had jaws.
Innovation, however, was afoot 450 million years ago. Some of these animals made a leap in lifestyle by adding the capability to capture and consume their neighbors. Jawed fish went on to spawn almost all the evolutionary branches of fish known today and every species of land animal with a backbone, including people.
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Jaws may not be our only biological legacy from those ancestors. New research is suggesting that another innovation arose around the same time–one more subtle than but just as pervasive as the jaw. These ancestors evolved a suite of proteins called receptors that recognize and respond to steroid hormones.
With this ancient version of the endocrine system, fish fine-tuned their physiology, using internal chemical signals to create differences between males and females and regulate reproduction. Learning how these receptors came about may eventually help scientists understand how descendants of the original receptors interact with chemicals in the modern world.
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A good innovation
Back in the early oceans, the innovation of having hormones and receptors was good enough to keep. All jawed fish and land vertebrates now share six steroid-hormone receptors. In contrast, no steroid-hormone receptor appears in insects and other invertebrates that branched away from our evolutionary line before our fishy ancestors did.
Steroid hormones act as messenger molecules in the body, traveling to various tissues and binding to receptors there. The receptors are like locks into which the hormones, such as estrogen, fit as keys. Once combined, the hormone-receptor complex stimulates processes such as ovulation, sperm production, the emergence of adult sexual characteristics, and changes in blood chemistry.
Estrogen and progesterone are called the female hormones because, among other things, they regulate female fertility. Together, they direct egg maturation, ovulation, and, in mammals, the events of pregnancy. Scientists are still discovering additional activities controlled by estrogen.
Researchers began investigating the roles of hormones and their receptors in the 1960s and 1970s. In the 1990s, modern genetic techniques identified the widely shared receptors: two for estrogen, one each for progesterone and androgen, which is often called the male hormone, and two for corticosteroids, which regulate metabolism.
Scientists still didn’t know how ancient animals jumped from zero steroid-hormone receptors to six. Even determining how the first of the hormone-receptor pairs evolved posed a problem. The simultaneous evolution of a lock and a key seems unlikely, say scientists. The two pieces would have had to evolve from two entities that didn’t originally go together and still fit precisely.
Most likely, one member of that first pair was close to its final form at the time the other began to evolve an affinity for it. If this is true, which evolved first, the hormone or the receptor?
“It’s a chicken-and-egg puzzle,” notes David P. Crews of the University of Texas at Austin.
Joseph W. Thornton of Columbia University’s Earth Institute and Department of Biological Sciences explains further that a hormone can’t act as a hormone without something to receive its signal. Similarly, a receptor can’t receive messages if there’s no hormone to signal it. It’s not obvious what function either of these would have without the other, he says.
Thornton reports in the May 8 Proceedings of the National Academy of Sciences that he has identified which steroid-hormone receptor evolved first in fishes. He also proposes a scenario for how the other steroid hormones and their receptors came about 450 million years ago.
Witness to a simpler time
Thornton revealed his target by studying a witness to that simpler time, before so many receptors muddied the waters.
Since all jawed vertebrates have the full complement of hormone receptors and no invertebrate does, the researchers focused on the root of the vertebrate family tree. Lampreys are modern, jawless descendants of those early fishes. Thornton compared the gene sequences of the lamprey with those of eels, quails, crocodiles, mice, cows, and other animals up and down the evolutionary tree.
Unlike all the others, lampreys have receptors for only three steroid hormones: estrogen, progesterone, and a corticoisteroid. Just as their body plan seems stuck in the past, lampreys’ complement of hormone receptors seems frozen in time.
Thornton reconstructed the ancestral hormone receptor by looking across the evolutionary tree at changes in the genes for the three early hormone receptors. He ended up with an estrogen receptor.
The finding makes estrogen “essentially the earth mother of all hormones,” remarks John A. McLachlan of Tulane and Xavier Universities in New Orleans.
While it’s not surprising that estrogen is one of the oldest hormones, says McLachlan, naming it “the first” turns a standard paradigm for receptor evolution on its head. In the biochemical assembly line, or pathway, for hormone synthesis, progesterone is made first and is followed by other hormones. Estrogen pops out at the end of the process.
A popular scenario re-creates evolution following this same pathway: Progesterone would be the first hormone to evolve and estrogen the last.
The receptors would arise after each hormone, making the progesterone receptor the most ancient and the estrogen receptor the last on the scene.
“As attractive as [this proposal] is, Thornton has provided an elegant and convincing challenge,” says Crews,
In this challenge, Thornton has hormone evolution begin at the end of the biochemical pathway. When estrogen became a hormone, it set the stage for the intermediate molecules along its assembly line to be enlisted as hormones themselves.
For unknown reasons, estrogen became linked to egg maturation in the early vertebrates, Thornton speculates. Once the hormone became a signal in the reproductive process, estrogen’s biochemical precursors in the pathway could become players in reproduction, as well.
Thornton points out that genes occasionally become duplicated in the genome. He suggests that if the estrogen-receptor gene was accidentally duplicated, one copy might have mutated slightly. Its protein might then suddenly have provided a match for one of the intermediary molecules in the pathway. Progesterone resembles estrogen more than the other hormones do and so is most likely to have become the second steroid hormone.
Thornton proposes that the gene duplication that created the progesterone receptor took place before the jawless fishes branched off the rest of the vertebrate line. Another duplication of either the estrogen or the progesterone receptor would have been necessary for lampreys to have their three steroid-hormone receptors.
A final wave of duplications in the vertebrate-only branch could have created the full set of six receptors. Despite its modern importance, the androgen receptor evolved in this later period of duplication, Thornton says. This would have happened among the jawed fish about 400 million years ago, before the line that eventually became four-legged creatures split away.
The new understanding of steroid hormones and receptors could lead pharmaceutical companies to investigate more intermediaries in the estrogen-forming pathway as molecules with hormone action, speculates Bert W. O’Malley of the Baylor College of Medicine in Houston.
A tough question
Thornton’s scenario leaves a tough question about the origin of steroid receptors. If the other five receptors branched from the estrogen-receptor pathway, where did the estrogen receptor come from?
O’Malley proposes that it may have derived from receptors that act on the cell’s nucleus. Such nuclear receptors are even more ancient than steroid receptors, and they bind to molecules that are simpler than steroid hormones. The molecular partners are still unknown for many nuclear receptors, which scientists therefore call orphans.
“My feeling is that the entire large superfamily [of steroid receptors and related nuclear receptors] evolved from one of the more primitive known orphans,” says O’Malley.
McLachlan takes a different view. In the June Endocrine Reviews, he compiles years of data on hormones that scientists have traced through embryonic and evolutionary development. In what he admits is speculation,
McLachlan suggests that the endocrine system in animals evolved from signals between organisms such as plants and bacteria.
Plants have estrogenlike chemicals, which are now called phytoestrogens, that have been around for hundreds of millions of years, McLachlan points out. Before steroid hormones were carrying signals within the bodies of animals, he suggests, phytoestrogens might have been signals between organisms. The first molecules in this biochemical family couldn’t have been hormones, he says.
Thornton’s idea that hormones arose from estrogen-synthesis intermediaries might apply also to the evolution of phytoestrogens. Bacteria or animal ancestors might have evolved receptors that detect these plant chemicals.
These organisms, with their ready-made receptors, may then have enlisted the signals for internal chemical-messaging systems, McLachlan proposes.
An example of these signals passes between alfalfa plants and the bacteria living in specialized root nodules that draw nitrogen from the soil for the plant and nourish the bacteria. Before a nodule forms, the alfalfa squirts out a protein similar to one of the hormonelike components of birth control pills. The root bacteria send back a signal saying, “We are about to go into your roots, but it’s okay,” says McLachlan. The two organisms then form the nodule.
Coral reefs may do something similar, says McLachlan. There is evidence that chemical signals pass between certain corals and their partner algae beds.
The ancient history of hormone receptors may have a bearing on a modern environmental problem. Over the past century, these receptors have proved vulnerable to a flood of industrial chemicals that can masquerade as hormones. They emanate from natural sources such as soybeans, as well as from decaying plastics, paper mills, and even the pharmaceutical-laden urine of people and farm animals (SN: 3/21/98, p. 187: http://www.sciencenews.org/sn_arc98/3_21_98/bob1.htm).
Thornton, who was once employed by the environmental activist organization Greenpeace, has studied such environmental endocrine disruption (SN: 3/1/97, p. 19: http://www.sciencenews.org/sn_arc97/75th/jr_essay.htm) for 2 decades.
Even a small accidental affinity of a receptor for a chemical similar to its hormone partner can be devastating, Thornton points out. The chemicals can either stimulate or block steroid-hormone receptors. Scientists have indicted the compounds for effects as diverse as turning male fish into females, accelerating girls’ puberty, and causing infertility in males exposed to the artificial estrogens in the womb.
The adverse effects may not be limited to animals. McLachlan’s lab is investigating whether environmental chemicals upset the bacteria-alfalfa relationship, paralleling endocrine disruption in animals.
Thornton’s findings help scientists estimate how widespread the effects of contamination might be from hormone-mimicking compounds. Many of the hormone mimics seem to act on the estrogen receptor. Thornton reasons that if it is the most ancient of the hormone receptors, then the vast majority of animal species–from worms and insects to fish and land vertebrates–may be affected by environmental estrogens.
To learn how environmental estrogens can fool a receptor, Crews and his colleagues are now trying to evolve hormone receptors from scratch in the lab. They plan then to investigate whether the novel receptors distinguish between an environmental and a natural estrogen.
Thornton has cracked open a window to a time hundreds of millions of years ago through which other scientists are now peeking. The result has been ” a whole new way of doing evolutionary endocrinology,” Crews says.