Analyzing the Scientiﬁc Evolution of the Sustainable Development Goals

: Development must balance social, economic, and environmental sustainability; it is for this reason that the Sustainable Development Goals (SDGs) are integrated, in fact, action in one of them will affect outcomes in others. In consequence, research on the SDGs is broad, complex, and fragmented due to the great diversity of disciplines and approaches involved, making it difﬁcult to obtain valuable and unbiased information for future studies. As a result, a comprehensive review of contributions could provide a comprehensive critical perspective. This article applies SciMAT software to analyze the evolution of this ﬁeld of research through a systematic literature review of bibliographic records on the SDGs and a review based on bibliometric analysis of 10,272 selected records. Additionally, hidden themes and their development in this ﬁeld from 1990 to 2020 have been identiﬁed to produce strategic diagrams, graphs of thematic evolution and performance indicators of the research ﬁeld in different periods. The results obtained show a constantly evolving scientiﬁc ﬁeld, from its initial focus on the millennium goals to the gradual inclusion of the current SDGs. They provide ﬁeld experts with a comprehensive overview of the status quo and predict the dynamic directions of future research, serving as a basis for the development of new strategies for the implementation of the SDGs.


Introduction
In 1987, in the Brundtland Report, the United Nations (UN) defined sustainable development as "development that meets the needs of the present without compromising the ability of future generations to meet their own needs" [1]. Furthermore, it emphasizes that the three interrelated concepts of sustainable development are social sustainability, economic sustainability, and environmental sustainability; this means that each action or decision must consider natural, human, and economic capital to achieve responsible and sustainable development [2,3]. As a result, over the past decades, the concept of sustainability has attracted global attention and has been widely discussed by policy-and decision-makers, academics, and experts. In fact, sustainable development is now part of almost all political agendas in which a variety of goals and targets have been set to make the world a more sustainable and resilient place for all.
Thus, the Millennium Summit that took place on 18 September 2000, in New York City, led to the adoption of the UN Millennium Declaration, which aimed to create an international association to reduce global levels of extreme poverty. This meeting was the result of work carried out over the course of 10 years. In the declaration, the 191 UN member states and world leaders defined values as well as guiding principles and committed to seven common goals: peace, security, and disarmament; development and poverty eradication; protection of our common environment; human rights, democracy, and individual articles or authors relate to each other [25], examining the bibliographic material from an objective and quantitative perspective [26].
Although studies have been found that apply biometric analysis in the field of SDGs research, they focus on specific disciplines. Thus, Pizzi et al. [27] systematically reviewed, through bibliometric methods and a systematic bibliographic review, scientific knowledge about the SDGs and the business sector. Prieto-Jiménez et al. [28] focused on SDG 4, quality education. Sousa Wands [29] provides a critical view of the role of plastic in the SDGs, discussing their pros and cons, outlining a future scenario based on bibliometric mapping, analysis, and recent literature. Belmonte-Ureña et al. [30] focused on the relationship between circular economy, degrowth, green growth and the SDGs. Rochon et al. [31] conducted a systematic review of the literature to provide information and lessons on how the SDGs statement can influence health financing reforms for universal health coverage based on countries' experiences.
For all the above, this article aims to analyze the existing knowledge about the SDGs and their evolution from the MDGs, using an integrated double analysis: a systematic review of the literature (SRL) and a bibliometric analysis. To meet this objective, the following specific objectives were established: (i) a quantitative analysis based on an SLR and (ii) a qualitative review based on performance analysis and scientific mapping. This study will contribute to the existing body of knowledge by highlighting and evaluating trends and patterns in SDGs research, establishing research topics, mapping researcher networks and finally recommending areas for future study.
This study will contribute to the existing body of knowledge by highlighting trends and patterns in the SDGs research field, establishing your research topics, mapping researcher networks, and recommending areas for future study.

Materials and Methods
A double analysis was performed to achieve the objectives of this study ( Figure 1). This double analysis consisted of the following phases: (i) an SRL of bibliographic records on SDGs and MDGs as well as (ii) a bibliometric analysis of the identified documents. Each of these phases is described in detail in the following sections.

Systematic Literature Review
The SRL establishes a methodological approach that collects all the evidence-based pre-specified eligibility criteria [32]. The SRL follows a protocol that defines the search string, the strategy, the exclusion criteria, and the methods to extract data by synthesizing the results [33]. In this article, SLR is based on the guidelines in Kitchenham et al. [34] ( Figure 1), which are collected in the following stages:

Bibliometric Analysis: Science Mapping and Performance Analysis
Once the SRL has been applied, a bibliometric analysis is performed. The bibliometric analysis aims to obtain a spatial representation of the relationship between disciplines, specializations, documents, and individual authors. It is an integrated double analysis: a performance analysis and a scientific mapping. The performance analysis evaluates the impact of the citations of the scientific production and the scientific mapping shows the conceptual, social, or intellectual structure of the research as well as its evolution and dynamic aspects. In this work, SciMAT v1.1.04 (Science Mapping Analysis Software Tool) was used to perform the bibliometric analysis, referring to the research topics.
SciMAT is based on the analysis of common words [36] and the h-index [37]. The hindex of a researcher measures the quality of their research based on the number of their articles in scientific journals and the citations received [38]. The h-index was defined by Hirsch [39] as "A scientist has an h-index if h of his articles Np has at least h citations each and the other articles (Np-h) have ≤ h citations each". SciMAT incorporates methods, algorithms, and measurements from all steps of the general scientific mapping workflow, from preprocessing to viewing scientific mapping results [38,39]. This tool establishes the stages described below.

Detection Research Themes and Creation of Two-Dimensional Strategic Diagrams
First, the tool generates the equivalence index to detect the research topics [40]. Then the software uses the single-center algorithm [41] to detect the most relevant topics. This generates a strategic diagram per set period. Diagrams are based on centrality and density [42]. Centrality is the degree of interaction of a research topic with other research topics. The density gives an idea of the level of development of that topic, measuring the internal

Bibliometric Analysis: Science Mapping and Performance Analysis
Once the SRL has been applied, a bibliometric analysis is performed. The bibliometric analysis aims to obtain a spatial representation of the relationship between disciplines, specializations, documents, and individual authors. It is an integrated double analysis: a performance analysis and a scientific mapping. The performance analysis evaluates the impact of the citations of the scientific production and the scientific mapping shows the conceptual, social, or intellectual structure of the research as well as its evolution and dynamic aspects. In this work, SciMAT v1.1.04 (Science Mapping Analysis Software Tool) was used to perform the bibliometric analysis, referring to the research topics.
SciMAT is based on the analysis of common words [36] and the h-index [37]. The h-index of a researcher measures the quality of their research based on the number of their articles in scientific journals and the citations received [38]. The h-index was defined by Hirsch [39] as "A scientist has an h-index if h of his articles Np has at least h citations each and the other articles (Np-h) have ≤ h citations each". SciMAT incorporates methods, algorithms, and measurements from all steps of the general scientific mapping workflow, from preprocessing to viewing scientific mapping results [38,39]. This tool establishes the stages described below.

Detection Research Themes and Creation of Two-Dimensional Strategic Diagrams
First, the tool generates the equivalence index to detect the research topics [40]. Then the software uses the single-center algorithm [41] to detect the most relevant topics. This generates a strategic diagram per set period. Diagrams are based on centrality and density [42]. Centrality is the degree of interaction of a research topic with other research topics. The density gives an idea of the level of development of that topic, measuring the internal cohesion of all the links between the keywords that describe the topic [43,44]. The diagrams are divided into four quadrants that capture the following four types of research topics: that are highly developed internally, but isolated from the rest of the topics. These are specialized topics in peripheral areas of the research field. (c) Emerging or declining issues. These are in the lower left quadrant. Topics that lack development and relevance. These topics can evolve and position themselves as relevant topics or disappear. These will be reflected in the next period. (d) Basic and transversal themes. These are in the lower right quadrant. These topics are essential to the scientific field, but they are not well developed yet.

Detection of the Thematic Networks
These graphs show the relationship of each topic of the strategy diagrams with the keywords and their interconnections. Figure 1b shows an example of a thematic network. Here, several keywords are interconnected, where the size of the circle is proportional to the number of documents corresponding to each keyword, and the thickness of the link between two circles is proportional to the equivalence index.

Detection of Conceptual Links between Research Themes
Through the inclusion index, conceptual links are detected between research topics in different periods [45]. The strength of association between themes is also detected. The following two types of graphics are used for its representation: • Overlay graphic. The horizontal arrow represents the number of words shared by both periods. The top entry arrow represents the number of new words in period 2 and the top exit arrow represents the missing words in period 2.

•
Thematic map of evolution. Solid lines mean that the linked topic shares the parent. A dotted line means that the topics share elements that are not the main element. The thickness of the borders is proportional to the inclusion index and the volume of the spheres is proportional to the number of published documents.

Performance Analysis
The relative contribution of the research topic to the entire research field is measured quantitatively and qualitatively, and is used to establish the most prominent, productive and high-impact subfields using bibliometric indicators, such as the number of articles and citations published and different types of h-index.

Results
Once the steps of the previous section have been applied the results are analyzed in Figures 2-7 and Tables 1-3 and are discussed below.

Systematic Literature Review
This section presents the results obtained by applying the SLR methodology. It includes the definition of the research questions, the search process, and the definition of the PRISMA flowchart ( Figure 2).

Planning and Problem Formulation
The main objective of this work is to analyze existing knowledge about the SDGs and their evolution since the MDGs. Therefore, the following research questions (RQ) were determined based on the objectives of this work: Regarding the selection of databases, in this study the Web of Science (WoS) and SCOPUS databases were selected. These databases have many high-impact international scientific and technical publications from all disciplines.
In terms of keywords, this review has addressed the evolution of the SDGs, which have their origin in the MDGs. Therefore, the keywords must cover both topics. Two search strings were used, one for each database. Thus, for SCOPUS, the search performed was TITLE-ABS-KEY ("SUSTAINABLE DEVELOPMENT GOALS") OR TITLE-ABS-KEY ("MILLENNIUM DEVELOPMENT GOALS"). For the WoS, the search was THEME: ("SUS-TAINABLE DEVELOPMENT GOALS") OR THEME: ("MILLENNIUM DEVELOPMENT GOALS"). The searches in the databases were carried out on 30 May 2021.

Selection of the Literature
Once the keywords and search strings were identified, the records obtained were collected and archived. Next, the PRISMA flowchart guide ( Figure 2) was applied, which shows the number of relevant documents identified. their evolution since the MDGs. Therefore, the following research questions (RQ) were determined based on the objectives of this work: Regarding the selection of databases, in this study the Web of Science (WoS) and SCOPUS databases were selected. These databases have many high-impact international scientific and technical publications from all disciplines.
In terms of keywords, this review has addressed the evolution of the SDGs, which have their origin in the MDGs. Therefore, the keywords must cover both topics. Two search strings were used, one for each database. Thus, for SCOPUS, the search performed was TITLE-ABS-KEY ("SUSTAINABLE DEVELOPMENT GOALS") OR TITLE-ABS-KEY ("MILLENNIUM DEVELOPMENT GOALS"). For the WoS, the search was THEME: ("SUSTAINABLE DEVELOPMENT GOALS") OR THEME: ("MILLENNIUM DEVELOP-MENT GOALS"). The searches in the databases were carried out on 30 May 2021.

Selection of the Literature
Once the keywords and search strings were identified, the records obtained were collected and archived. Next, the PRISMA flowchart guide ( Figure 2) was applied, which shows the number of relevant documents identified. A total of 30,597 bibliographic records were retrieved from the two selected databases. After eliminating 16,487 duplicates, 1769 of the remaining 14,111 records were excluded, applying the exclusion criteria (only article-type documents, full documents, and documents in English). The remaining 12,342 records were then screened for title, abstract and keywords, and an additional 1338 records were excluded because they did not cover the topics included in this review. Finally, after a final exclusion phase, 10,272 relevant documents were selected for the study.

Identification of Periods
The identification of the periods was determined from the main milestones and turning points in the evolution of the SDGs and MDGs. The time horizon used was from 1990 to 2020, since the inclusion in the scientific literature of the fundamental issues from which the SDGs are derived took place in the 1990s. It was then subdivided into the following three periods, considering the number of selected papers, as well as the relevant milestones, to analyze trends in publication patterns:

Bibliometric Analysis: Science Mapping and Performance Analysis
In this section, the following graphs are analyzed and discussed: strategic diagrams, one for each period; thematic networks of relevant topics; and an overlay chart and thematic evolution map. In addition, the most cited documents and publications are analyzed.

Bibliometric Analysis: Science Mapping and Performance Analysis
In this section, the following graphs are analyzed and discussed: strategic diagrams, one for each period; thematic networks of relevant topics; and an overlay chart and thematic evolution map. In addition, the most cited documents and publications are analyzed.

Strategic Diagrams
Three strategic diagrams (shown in Figure 4) were generated to analyze the changes over time in the research field for the three periods considered (1990-2010, 2011-2015 and 2016-2020). Table 1 shows the performance measures obtained for each topic in terms of several documents, h-index and values of centrality and density. Below is an analysis of the results obtained for each period.

•
First period (1990-2010). According to the strategic diagram presented in Figure 4a, 12 research topics are observed in the 1171 selected works in this period. Of these, five are driving themes (rural area, human, Millennium Development Goals, young adult, and health care facility, three are emerging or in decline (human rights, water resource and practice guideline), two are highly developed and isolated (United Nations and environmental policy) and two are considered basic and cross-cutting issues (economics and government). The performance analysis of each topic, from Table 1, complements the information provided by the diagram. The topics with the highest performance measures are human, health care facility, economics, and government. These topics have more than 25,000 citations between them, obtaining a higher h-index than the rest of the topics. • Second period (2011)(2012)(2013)(2014)(2015). According to the strategic diagram presented in Figure 4b, 16 themes are observed in the 2148 works selected in this period. Six of these issues were driving issues (HIV infections, financial management, rural area, Millennium Development Goals, health policy and human), four were emerging or declining themes (environment, United Nations, climate change and statistical model), three were highly developed and isolated themes (mobile phone, household survey and non-communicable disease) and three were basic and cross-cutting themes (retrospective study, trends and health care system). According to the performance measures (Table 1), the following three themes can be highlighted: human, health policy and cross-sectional study. These research topics had a high h-index and were considered in the largest number of documents. • Third period (2016-2020). According to the strategic scheme presented in Figure 4c, nine research topics are observed in the 6953 works selected in this period. Seven were driving themes (greenhouse gases, disability-adjusted life year, health care cost, maternal health service, Sustainable Development Goals, human and health policy), six were emerging or declining themes (health risk, innovation, environmental impact assessment, prospective study, investment, innovation, and water resource) and two were basic topics (household and retrospective study). According to the performance measures (Table 1), the topics Sustainable Development Goals, human and health policy can be highlighted. These research topics had a high impact and had a higher h-index than the other topics. 1

Thematic Networks
In this section, to further explore relevant themes and future trends, three thematic networks, one per period, were selected based on their saliency, which captures developed, basic and emerging research over time. The interconnections between the themes (circles) are represented in Figure 5. The circle size indicates the number of documents, and the thickness of the line corresponds to the strength of the correlation between the two nodes. An analysis of the results obtained for each period is shown below. In Figure 5, we can see that the Millennium Development Goals topic is closely linked to other essential topics such as low-income population, rural area, United Nations, water resource, aid flow, climate change, economic growth, education policy and health impact,

Thematic Networks
In this section, to further explore relevant themes and future trends, three thematic networks, one per period, were selected based on their saliency, which captures developed, basic and emerging research over time. The interconnections between the themes (circles) are represented in Figure 5. The circle size indicates the number of documents, and the thickness of the line corresponds to the strength of the correlation between the two nodes. An analysis of the results obtained for each period is shown below.
In Figure 5, we can see that the Millennium Development Goals topic is closely linked to other essential topics such as low-income population, rural area, United Nations, water resource, aid flow, climate change, economic growth, education policy and health impact, among others. Thus, in the years that preceded the introduction of the SDGs, the international development agenda focused on the economic performance of countries. The MDGs incorporated a greater focus on economic development. However, it was believed that such development would be achieved mainly through the global eradication of extreme poverty [45][46][47]. The MDGs and their founding document, the Millennium Declaration, focused on reducing economic poverty without a clear focus on other dimensions of development such as human rights, empowerment, and equality [48]. Therefore, poverty reduction was framed primarily as something that could be addressed through economic rather than social or political reforms [49,50].
On the other hand, as seen in Figure 6, the SDG issue is related to issues such as stakeholder, United Nations, economic growth, human rights, integrated approach, and policy implementation, among others. This shows that the SDGs differ in several key elements compared to the MDGs. The SDGs expanded the goal of the MDGs, that is, the reduction of extreme poverty, to include four dimensions: inclusive social development, inclusive economic development, environmental sustainability and peace and security. The SDGs take a much more comprehensive approach to sustainable development than the MDGs. They offer a more people-centered development agenda. In general, the SDGs present a much more integrated approach to sustainable development than the MDGs [51][52][53]. Unlike the MDGs that focused on developing countries, the SDGs apply to all development challenges regardless of country. Developing and developed countries are all responsible for sustainable development through the implementation of the SDGs [54].

Overlay Graph and Thematic Evolution Map
Finally, to analyze the evolution of the topics over time, Figure 7a,b represents the number of keywords per period and their evolution, the number of incoming and outgoing keywords in addition to the number and percent of keywords that are used. They persist from one period to the next. As seen in Figure 7a, the number of keywords grows over the periods, in parallel with the increase in the number of documents over the years. The number of keywords increases from 209 to 1625 between the first and the last period, a growth rate of 123%. Specifically, of the 209 keywords that appeared in the first period, 93% (195) remain in the second period, and 837 new words are added, for a total of 1032 words. Finally, in the third period, 965 (94%) keywords remain from the second period and 730 new keywords appear, resulting in a total of 1696. These results indicate that the number of new and transitional keywords is high, but also that the number of keywords

Overlay Graph and Thematic Evolution Map
Finally, to analyze the evolution of the topics over time, Figure 7a,b represents the number of keywords per period and their evolution, the number of incoming and outgoing keywords in addition to the number and percent of keywords that are used. They persist from one period to the next. As seen in Figure 7a, the number of keywords grows over the periods, in parallel with the increase in the number of documents over the years. The number of keywords increases from 209 to 1625 between the first and the last period, a growth rate of 123%. Specifically, of the 209 keywords that appeared in the first period, 93% (195) remain in the second period, and 837 new words are added, for a total of 1032 words. Finally, in the third period, 965 (94%) keywords remain from the second period and 730 new keywords appear, resulting in a total of 1696. These results indicate that the number of new and transitional keywords is high, but also that the number of keywords shared for successive periods has increased. Therefore, the growing thematic diversity of the MDGs and SDGs research field and the fact that keywords reappear with greater force in the following periods could be indicators that this research field is gradually consolidating, but also that the number of shared keywords has increased for successive periods. Therefore, the growing thematic diversity of the MDGs and SDGs research field as well as the fact that keywords reappear with greater force in the following periods could be indicators that this research field is gradually consolidating but also that the number of shared keywords has increased for successive periods. Therefore, the growing thematic diversity of the MDG and SDG research field and the fact that keywords reappear with greater force in the following periods could be indicators that this research field is gradually consolidating.

Performance Analysis
The 10,272 documents analyzed received 24,591 citations. Table 2 shows the five publications with the highest number of citations, a total of 3052, which represents 12% of the total. The most-cited documents focus on different aspects of the thematic field analyzed, revealing its diversity. These papers deal with the goals, indicators, values, and practice of sustainable development; soil science and how the discipline is related to land has important links to several of the SDGs, which are demonstrated through the functions of soils and the ecosystem services that are linked to those functions [55][56][57][58]. Additionally, analyzing progress on specific topics towards MDGs four and five started in 2010 with additional surveys, censuses, vital records, and verbal autopsy data [56]. Furthermore, for the third Millennium Development Goal (MDG), on gender equality and the empowerment of women, exploring the concept of women's empowerment and highlighting the indicators associated with this goal (education, employment, and political participation) can contribute to it [57].
In this study, 2663 journals were identified. Table 3 shows the publications of 18.01% of the analyzed documents, which are arranged in descending order according to the number of documents. Most are research journals that focus on human-environmental, cultural, economic, and social sustainability, cleaner production practices and development studies.
Finally, Figure 7b shows the thematic evolution of the research field through the analysis of the origins and interrelationships of the themes. The thickness of the lines represents the strength of the association as measured by the inclusion index. If the graph is analyzed from the point of view of the number of documents, the environmental policy theme of the first period evolved into the climate change, rural area, and household survey themes for the second period. For the first period, it is worth highlighting the economics theme, which evolved into the human, health policy, HIV infections and statistical model themes.
additional surveys, censuses, vital records, and verbal autopsy data [56]. Furthermore, for the third Millennium Development Goal (MDG), on gender equality and the empowerment of women, exploring the concept of women's empowerment and highlighting the indicators associated with this goal (education, employment, and political participation) can contribute to it [57].   The significance of soils and soil science towards the realization of the United Nations sustainable development goals

Discussion
As can be seen from the results, the MDGs were revolutionary because they offered a common discourse to reach a global agreement. The eight objectives were realistic, easy to communicate and had a clear mechanism for quantifying and monitoring them. The progress made on the MDGs was substantial. When the Millennium Development Goals (MDGs) were established in 2000, many authors argued that their goals were so ambitious that they could not be achieved. Within this framework of achievements and progress came the Sustainable Development Goals (SDGs) and the 2030 Agenda, a new roadmap that represents an opportunity to incorporate the lessons learned in the previous process and reach children who, for various reasons, have been left out of progress and development, and focus on building a sustainable world in which environmental sustainability, social inclusion and economic development are equally valued.
By 2015, the world was already consistent in reducing the world rates of extreme poverty and hunger by half, and with the arrival of the SDGs the momentum generated by the MDGs continued within a global development framework that had people as its central axis. Therefore, organizations, citizens, scientists, academics, and representatives of the private sector from all over the world were protagonists in the process of developing the new program: a framework document that will govern the activities for the well-being of people and the care of the planet for the next 15 years.
The discussions that have taken place since the launch of the SDGs tend to focus on how society can contribute to achieving these goals. However, it is often very easily taken for granted that society is already convinced that they should do so.

Conclusions
This article offered a comprehensive discursive analysis of the transition from the MDGs to the SDGs, which until now has only been partially done. The literature review shows that discourses around sustainable development have evolved with the transition from the MDGs to the SDGs. The SDGs have benefited from both the successes and the shortcomings of the MDGs. The MDGs have had their share of weaknesses, which have been recognized to improve the performance of the SDGs.
Thus, the Millennium Development Goals (MDGs) mark a historic and effective method of global mobilization to achieve a set of important social priorities throughout the world. They express widespread public concern about poverty, hunger, disease, gender inequality and environmental degradation. The MDGs help promote global awareness, political responsibility, improved metrics, social feedback, and public pressure.
Developing countries have made substantial progress towards achieving the MDGs, although progress varies widely across targets, countries, and regions. Furthermore, for more than a decade, the MDGs have remained at the center of global policy debates and national policy planning. They have been incorporated into the work of non-governmental organizations and civil society in general and are taught to students at all levels of education.
Furthermore, there is a general feeling among policymakers and civil society that progress against poverty, hunger and disease is remarkable, and that the MDGs have played an important role in ensuring that progress. In a world already suffering from dangerous climate change and other serious environmental ills, there is also a widespread understanding that global environmental goals need a higher profile alongside poverty reduction goals.
On the other hand, the evolution towards the SDGs finally helps to take the world onto a sustainable path. The results show how the SDGs have rapidly gained ground due to the growing urgency of sustainable development for the whole world. These show a combination of economic development, environmental sustainability, and social inclusion. Certainly, thus far, no consensus has been reached on trade-offs and synergies between economic, environmental, and social objectives. Still, a shared focus on economic, environmental, and social goals is a hallmark of sustainable development and represents a broad consensus on which the world can build.